Interim Financial Data by Segment (Financial Data By Strategic Business Unit Segment) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Jun. 30, 2019
Segment Reporting Information [Line Items]
Revenues	$ (3,495.7)	$ (3,310.3)	$ (14,110.2)
Earnings before Income Taxes	739.1	646.8	$ 3,005.6
Employer Services
Segment Reporting Information [Line Items]
Revenues	(2,441.4)	(2,338.2)
Earnings before Income Taxes	681.0	639.8
PEO Services
Segment Reporting Information [Line Items]
Revenues	(1,056.9)	(974.9)
Earnings before Income Taxes	148.4	143.7
Other
Segment Reporting Information [Line Items]
Revenues	(2.6)	(2.8)
Earnings before Income Taxes	$ (90.3)	$ (136.7)