Interim Financial Data by Segment (Financial Data By Strategic Business Unit Segment) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Segment Reporting Information [Line Items]
Revenues	$ 3,695.7	$ 3,669.5	$ 7,166.5	$ 7,165.2
Earnings before Income Taxes	832.6	835.5	1,597.8	1,574.6
Employer Services
Segment Reporting Information [Line Items]
Revenues	2,510.6	2,537.5	4,887.4	4,978.9
Earnings before Income Taxes	758.6	766.3	1,447.8	1,445.7
PEO Services
Segment Reporting Information [Line Items]
Revenues	1,186.1	1,134.7	2,282.0	2,191.5
Earnings before Income Taxes	187.4	168.0	346.8	317.3
Other
Segment Reporting Information [Line Items]
Revenues	(1.0)	(2.7)	(2.9)	(5.2)
Earnings before Income Taxes	$ (113.4)	$ (98.8)	$ (196.8)	$ (188.4)