Quarterly report pursuant to Section 13 or 15(d)

Description of Business, Basis of Presentation, and Significant Accounting Policies (Narrative) (Details)

v3.19.3
Description of Business, Basis of Presentation, and Significant Accounting Policies (Narrative) (Details)
$ in Millions
 3 Months Ended 12 Months Ended
Jun. 01, 2019
USD ($)
Aug. 31, 2019
USD ($)
segment
Aug. 31, 2018
USD ($)
Jun. 30, 2020
USD ($)
May 31, 2020
USD ($)
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Jun. 30, 2019
USD ($)
May 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
May 31, 2018
USD ($)
Accounting Policies [Line Items]                        
Number of reportable segments | segment   1                    
Net cash provided by investing activities   $ 544.7 $ 156.0 [1]                  
Cash, cash equivalents, restricted cash and restricted cash equivalents   1,361.5 1,619.9 [1]           $ 935.2     $ 2,300.5 [1]
Cash and cash equivalents   586.4 440.1 [1]           673.6      
Allowance for doubtful accounts   10.6             7.5      
Advance collections   4.2             4.2      
Maximum individual workers' compensation claims liability                 1.0      
Maximum individual health insurance claims liability                   $ 0.3    
Operating lease, impairment loss   0.0                    
Stock-based compensation costs recognized   10.5 10.9                  
Operating lease, right-of-use asset   112.8                    
Operating lease, liability   130.8                    
Forecast [Member]                        
Accounting Policies [Line Items]                        
Maximum individual workers' compensation claims liability         $ 1.0              
Maximum individual health insurance claims liability           $ 0.3            
ASU 2016-18 [Member]                        
Accounting Policies [Line Items]                        
Net cash provided by investing activities     (762.5)                  
Cash, cash equivalents, restricted cash and restricted cash equivalents     $ 1,200.0                 $ 1,900.0
ASU 2016-02 [Member]                        
Accounting Policies [Line Items]                        
Increase in lease-related assets and liabilities $ 116.4                      
Operating lease, right-of-use asset 116.4                      
Operating lease, liability 135.3                      
Unamortized landlord allowances and lease incentives $ 18.9                      
Trade Receivables [Member]                        
Accounting Policies [Line Items]                        
Receivables   77.2             94.5      
Purchased Receivables [Member]                        
Accounting Policies [Line Items]                        
Receivables   $ 358.5             333.5      
Oasis And HROi [Member] | Forecast [Member]                        
Accounting Policies [Line Items]                        
Maximum individual workers' compensation claims liability         $ 1.0              
Oasis Outsourcing Group Holdings, L.P. [Member]                        
Accounting Policies [Line Items]                        
Maximum individual workers' compensation claims liability                 $ 1.0      
HR Outsourcing Holdings, Inc. [Member]                        
Accounting Policies [Line Items]                        
Maximum individual workers' compensation claims liability               $ 0.3     $ 0.5  
HR Outsourcing Holdings, Inc. [Member] | Subsequent Event [Member]                        
Accounting Policies [Line Items]                        
Maximum individual workers' compensation claims liability             $ 0.8          
HR Outsourcing Holdings, Inc. [Member] | Forecast [Member]                        
Accounting Policies [Line Items]                        
Maximum individual workers' compensation claims liability       $ 0.3                
Europe [Member] | Revenue [Member] | Geographic Concentration Risk [Member]                        
Accounting Policies [Line Items]                        
Total percentage   1.00% 1.00%                  
Europe [Member] | Long Lived Assets [Member] | Geographic Concentration Risk [Member]                        
Accounting Policies [Line Items]                        
Total percentage   5.00%             5.00%      
[1] Amounts have been adjusted to reflect adoption of Accounting Standards Update (“ASU”) No. 2016-18, “Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force).”