Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements (Details)

v3.3.1.900
Fair Value Measurements (Details) - USD ($)
$ in Millions
Nov. 30, 2015
May. 31, 2015
Available-for-sale securities:    
Available-for-sale securities $ 2,917.3 $ 3,595.6
Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents 118.7 103.1
Available-for-sale securities:    
Available-for-sale securities 2,917.3 3,595.6
Other 15.3 14.2
Liabilities:    
Other long-term liabilities 15.3 14.2
Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents 0.0 0.2
Available-for-sale securities:    
Available-for-sale securities 0.0 0.0
Other 15.3 14.2
Liabilities:    
Other long-term liabilities 15.3 14.2
Significant other observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents 118.7 102.9
Available-for-sale securities:    
Available-for-sale securities 2,917.3 3,595.6
Other 0.0 0.0
Liabilities:    
Other long-term liabilities 0.0 0.0
Significant unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents 0.0 0.0
Available-for-sale securities:    
Available-for-sale securities 0.0 0.0
Other 0.0 0.0
Liabilities:    
Other long-term liabilities 0.0 0.0
Commercial paper [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents 112.4 15.0
Commercial paper [Member] | Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents 0.0 0.0
Commercial paper [Member] | Significant other observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents 112.4 15.0
Commercial paper [Member] | Significant unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents 0.0 0.0
General obligation municipal bonds [Member]    
Available-for-sale securities:    
Available-for-sale securities 1,697.4 1,703.7
General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents 6.3 55.1
Available-for-sale securities:    
Available-for-sale securities 1,697.4 1,703.7
General obligation municipal bonds [Member] | Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents 0.0 0.0
Available-for-sale securities:    
Available-for-sale securities 0.0 0.0
General obligation municipal bonds [Member] | Significant other observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents 6.3 55.1
Available-for-sale securities:    
Available-for-sale securities 1,697.4 1,703.7
General obligation municipal bonds [Member] | Significant unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents 0.0 0.0
Available-for-sale securities:    
Available-for-sale securities 0.0 0.0
Money market securities [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents   0.2
Money market securities [Member] | Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents   0.2
Money market securities [Member] | Significant other observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents   0.0
Money market securities [Member] | Significant unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents   0.0
Corporate bonds [Member]    
Available-for-sale securities:    
Available-for-sale securities 87.7  
Corporate bonds [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale securities:    
Available-for-sale securities 87.7  
Corporate bonds [Member] | Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale securities:    
Available-for-sale securities 0.0  
Corporate bonds [Member] | Significant other observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale securities:    
Available-for-sale securities 87.7  
Corporate bonds [Member] | Significant unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale securities:    
Available-for-sale securities 0.0  
Pre-refunded municipal bonds [Member]    
Available-for-sale securities:    
Available-for-sale securities 86.2 102.7
Pre-refunded municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents   20.5
Available-for-sale securities:    
Available-for-sale securities 86.2 102.7
Pre-refunded municipal bonds [Member] | Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents   0.0
Available-for-sale securities:    
Available-for-sale securities 0.0 0.0
Pre-refunded municipal bonds [Member] | Significant other observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents   20.5
Available-for-sale securities:    
Available-for-sale securities 86.2 102.7
Pre-refunded municipal bonds [Member] | Significant unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents   0.0
Available-for-sale securities:    
Available-for-sale securities 0.0 0.0
Revenue municipal bonds [Member]    
Available-for-sale securities:    
Available-for-sale securities 943.3 963.6
Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents   12.3
Available-for-sale securities:    
Available-for-sale securities 943.3 963.6
Revenue municipal bonds [Member] | Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents   0.0
Available-for-sale securities:    
Available-for-sale securities 0.0 0.0
Revenue municipal bonds [Member] | Significant other observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents   12.3
Available-for-sale securities:    
Available-for-sale securities 943.3 963.6
Revenue municipal bonds [Member] | Significant unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Assets:    
Cash Equivalents   0.0
Available-for-sale securities:    
Available-for-sale securities 0.0 0.0
U.S. government agency securities [Member]    
Available-for-sale securities:    
Available-for-sale securities 88.0  
U.S. government agency securities [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale securities:    
Available-for-sale securities 88.0  
U.S. government agency securities [Member] | Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale securities:    
Available-for-sale securities 0.0  
U.S. government agency securities [Member] | Significant other observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale securities:    
Available-for-sale securities 88.0  
U.S. government agency securities [Member] | Significant unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale securities:    
Available-for-sale securities 0.0  
Variable rate demand notes [Member]    
Available-for-sale securities:    
Available-for-sale securities 14.7 825.6
Variable rate demand notes [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale securities:    
Available-for-sale securities 14.7 825.6
Variable rate demand notes [Member] | Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale securities:    
Available-for-sale securities 0.0 0.0
Variable rate demand notes [Member] | Significant other observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale securities:    
Available-for-sale securities 14.7 825.6
Variable rate demand notes [Member] | Significant unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]    
Available-for-sale securities:    
Available-for-sale securities $ 0.0 $ 0.0