Interim Financial Data by Segment (Financial Data By Strategic Business Unit Segment) (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Revenues	$ 4,668.0	$ 4,391.0	$ 9,180.3	$ 8,606.6
Earnings before Income Taxes	1,143.8	1,058.1	2,236.6	2,068.1
Employer Services
Segment Reporting Information [Line Items]
Revenues	3,125.2	2,891.9	6,171.6	5,682.2
Earnings before Income Taxes	1,061.9	934.6	2,070.4	1,798.1
PEO Services
Segment Reporting Information [Line Items]
Revenues	1,546.1	1,503.3	3,015.7	2,932.7
Earnings before Income Taxes	255.0	255.6	477.9	485.9
Other
Segment Reporting Information [Line Items]
Revenues	(3.3)	(4.2)	(7.0)	(8.3)
Earnings before Income Taxes	$ (173.1)	$ (132.1)	$ (311.7)	$ (215.9)