Basis of Presentation PEO Revenues (Details) - USD ($) $ in Millions		3 Months Ended		12 Months Ended
		Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019
Error Corrections and Prior Period Adjustments Restatement [Line Items]
Revenues, Net		$ 2,306.2	$ 2,218.6
Interest On Funds Held Clients		133.9	118.5
P E O Revenues	[1]	1,055.6	973.2	$ 4,172.5
Revenues		3,495.7	3,310.3	14,110.2
Operating Costs and Expenses		1,787.7	1,697.0	7,080.9
Systems development programming costs		168.2	158.0
Cost, Depreciation and Amortization		88.9	72.6
Selling, General and Administrative Expense		726.5	713.9
Interest Expense		39.9	35.9
Costs and Expenses		2,811.2	2,677.4	11,215.7
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest		739.1	646.8	3,005.6
Income Tax Expense (Benefit)		156.7	141.4	712.8
Net Income (Loss) Attributable to Parent		$ 582.4	505.4	2,292.8
Restatement Adjustment [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
P E O Revenues	[1]		(12.9)	(65.0)
Revenues			(12.9)	(65.0)
Operating Costs and Expenses			(12.9)	(65.0)
Costs and Expenses			(12.9)	(65.0)
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest			0.0	0.0
Income Tax Expense (Benefit)			0.0	0.0
Net Income (Loss) Attributable to Parent			0.0	0.0
Previously Reported [Member]
Error Corrections and Prior Period Adjustments Restatement [Line Items]
P E O Revenues	[1]		986.1	4,237.5
Revenues			3,323.2	14,175.2
Operating Costs and Expenses			1,709.9	7,145.9
Costs and Expenses			2,690.3	11,280.7
Income (Loss) from Continuing Operations before Equity Method Investments, Income Taxes, Noncontrolling Interest			646.8	3,005.6
Income Tax Expense (Benefit)			141.4	712.8
Net Income (Loss) Attributable to Parent			$ 505.4	$ 2,292.8

[1] Professional Employer Organization (“PEO”) revenues are net of direct pass-through costs, primarily consisting of payroll wages and payroll taxes of $10,510.6 million and $9,629.4 million for the three months ended September 30, 2019 and 2018, respectively.