Annual report pursuant to Section 13 and 15(d)

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

v3.19.2
Description of Business, Basis of Presentation, and Significant Accounting Policies (Narrative) (Details)
$ / shares in Units, shares in Millions, $ in Millions
 12 Months Ended
Jul. 11, 2019
$ / shares
Jun. 01, 2019
USD ($)
shares
Dec. 31, 2019
USD ($)
Sep. 30, 2019
USD ($)
Jun. 30, 2019
USD ($)
May 31, 2019
USD ($)
segment
Dec. 31, 2018
USD ($)
Sep. 30, 2018
USD ($)
Jun. 30, 2018
USD ($)
May 31, 2018
USD ($)
May 31, 2017
USD ($)
Dec. 20, 2018
USD ($)
Aug. 18, 2017
USD ($)
Jun. 01, 2016
USD ($)
Accounting Policies [Line Items]                            
Number of reportable segments | segment           1                
Cash and cash equivalents           $ 673.6       $ 358.2 [1],[2],[3] $ 184.6 [2],[3]      
Allowance for doubtful accounts           7.5       7.0        
Advance collections           4.2       17.2        
Goodwill           1,782.6       814.0 [1] 657.1      
Goodwill impairment loss           0.0       0.0 0.0      
Impairment of long-lived assets           0.0       0.0 0.0      
Realized gains or losses from foreign exchange transactions           0.0       0.0 0.0      
Maximum individual workers' compensation claims liability           1.0       1.3        
Workers' compensation insurance costs in current liabilities           71.1       19.4        
Workers' compensation insurance costs in long-term liabilities           99.2       31.2        
Maximum individual health insurance claims liability     $ 0.3       $ 0.3              
PEO accrued health insurance and dental and vision plan reserves in current liabilities           25.4       25.4        
Reserve for uncertain tax positions           21.6       14.7        
Cumulative adjustment to increase stockholder's equity                     $ 262.9      
Subsequent Event [Member]                            
Accounting Policies [Line Items]                            
Declared date Jul. 11, 2019                          
Quarterly dividend per share | $ / shares $ 0.62                          
Payable date Aug. 22, 2019                          
Record date Aug. 01, 2019                          
Common stock purchased, shares | shares   1.0                        
Common stock purchased, value   $ 84.0                        
ASU 2014-09 [Member]                            
Accounting Policies [Line Items]                            
Cumulative adjustment to increase stockholder's equity                           $ 262.9
Trade Receivables [Member]                            
Accounting Policies [Line Items]                            
Receivables           87.0       107.6        
Purchased Receivables [Member]                            
Accounting Policies [Line Items]                            
Receivables           $ 333.5       $ 267.0        
Software Development [Member]                            
Accounting Policies [Line Items]                            
Estimated useful lives           12 years                
Minimum [Member]                            
Accounting Policies [Line Items]                            
Payroll funding fees average collection period           35 days                
Minimum [Member] | ASU 2016-02 [Member] | Subsequent Event [Member]                            
Accounting Policies [Line Items]                            
Estimates increase in lease-related assets and liabilities   125.0                        
Minimum [Member] | Finite Lived Assets [Member]                            
Accounting Policies [Line Items]                            
Amortization period           3 years                
Minimum [Member] | Software [Member]                            
Accounting Policies [Line Items]                            
Estimated useful lives           3 years                
Maximum [Member]                            
Accounting Policies [Line Items]                            
Short-term investments maturity at acquisition           90 days                
Payroll funding fees average collection period           45 days                
Maximum [Member] | ASU 2016-02 [Member] | Subsequent Event [Member]                            
Accounting Policies [Line Items]                            
Estimates increase in lease-related assets and liabilities   $ 135.0                        
Maximum [Member] | Finite Lived Assets [Member]                            
Accounting Policies [Line Items]                            
Amortization period           12 years                
Maximum [Member] | Software [Member]                            
Accounting Policies [Line Items]                            
Estimated useful lives           5 years                
Oasis Outsourcing Group Holdings, L.P. [Member]                            
Accounting Policies [Line Items]                            
Goodwill                       $ 972.1    
Maximum individual workers' compensation claims liability           $ 1.0                
HR Outsourcing Holdings, Inc. [Member]                            
Accounting Policies [Line Items]                            
Goodwill                         $ 51.1  
Maximum individual workers' compensation claims liability       $ 0.8       $ 0.5            
Maximum individual health insurance claims liability         $ 0.3       $ 0.3          
Europe [Member] | Revenue [Member] | Geographic Concentration Risk [Member]                            
Accounting Policies [Line Items]                            
Total percentage           1.00%                
Europe [Member] | Revenue [Member] | Geographic Concentration Risk [Member] | Maximum [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%       10.00%        
PEO And Insurance Services [Member]                            
Accounting Policies [Line Items]                            
Cash and cash equivalents           $ 178.8       $ 44.2        
[1] Amounts have been adjusted to reflect the adoption of ASC Topic 606 and to provide further detail related to certain professional employer organization ("PEO") balances.
[2] Amounts have been adjusted to reflect the adoption of ASC Topic 606.
[3] Amounts have been adjusted to reflect the 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)."