Basis of Presentation (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Error Corrections and Prior Period Adjustments Restatement [Line Items]
PEO revenues	[1]	$ 1,133.4	$ 1,043.5	$ 2,189.0	$ 2,017.3
TOTAL REVENUES		3,669.5	3,492.4	7,165.2	6,802.7
Operating expenses		1,836.0	1,772.4	3,623.6	3,469.5
Total Expenses		2,880.0	2,784.0	5,691.2	5,461.4
EARNINGS BEFORE INCOME TAXES		835.5	741.0	1,574.6	1,387.8
Provision for income taxes		183.9	182.8	340.6	324.2
NET EARNINGS		$ 651.6	558.2	$ 1,234.0	1,063.6
Revision
Error Corrections and Prior Period Adjustments Restatement [Line Items]
PEO revenues	[1]		(13.5)		(26.4)
TOTAL REVENUES			(13.5)		(26.4)
Operating expenses			(13.5)		(26.4)
Total Expenses			(13.5)		(26.4)
EARNINGS BEFORE INCOME TAXES			0.0		0.0
Provision for income taxes			0.0		0.0
NET EARNINGS			0.0		0.0
As reported
Error Corrections and Prior Period Adjustments Restatement [Line Items]
PEO revenues	[1]		1,057.0		2,043.7
TOTAL REVENUES			3,505.9		6,829.1
Operating expenses			1,785.9		3,495.9
Total Expenses			2,797.5		5,487.8
EARNINGS BEFORE INCOME TAXES			741.0		1,387.8
Provision for income taxes			182.8		324.2
NET EARNINGS			$ 558.2		$ 1,063.6

[1] Professional Employer Organization (“PEO”) revenues are net of direct pass-through costs, primarily consisting of payroll wages and payroll taxes of $12,371.6 million and $11,751.1 million for the three months ended December 31, 2019 and 2018, respectively, and $22,882.2 million and $21,380.5 million for the six months ended December 31, 2019 and 2018, respectively.