Annual report pursuant to Section 13 and 15(d)

Commitments and Contingencies (Details)

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Commitments and Contingencies (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
May. 31, 2015
Feb. 28, 2015
Nov. 30, 2014
Aug. 31, 2014
May. 31, 2015
Line of Credit [Member] | JP Morgan Chase Bank, N.A. [Member]          
Lines of credit          
Amount available $ 350.0       $ 350.0
Expiration date         Feb. 28, 2016
Line of Credit [Member] | Bank of America, N.A. [Member]          
Lines of credit          
Amount available 250.0       $ 250.0
Expiration date         Feb. 28, 2016
Line of Credit [Member] | PNC Bank, National Association [Member]          
Lines of credit          
Amount available 150.0       $ 150.0
Expiration date         Feb. 28, 2016
Line of Credit [Member] | Wells Fargo Bank, National Association [Member]          
Lines of credit          
Amount available 150.0       $ 150.0
Expiration date         Feb. 28, 2016
Revolving Credit Facility [Member]          
Lines of credit          
Expiration date         Jun. 21, 2018
Maximum Amount Outstanding During Period $ 0.0 $ 0.0 $ 150.0 $ 100.0  
Interest Rate on amount borrowed 0.00% 0.00% 3.25% 3.25%