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).”