Interim Financial Data by Segment - Financial Data By Strategic Business Unit Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2024	Dec. 31, 2023
Segment Reporting Information [Line Items]
Revenues	$ 5,048.4	$ 4,668.0	$ 9,881.1	$ 9,180.3
Earnings before Income Taxes	1,261.3	1,143.8	2,497.5	2,236.6
Other
Segment Reporting Information [Line Items]
Revenues	(3.4)	(3.3)	(6.2)	(7.0)
Earnings before Income Taxes	(173.4)	(173.1)	(327.1)	(311.7)
Employer Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	3,388.5	3,125.2	6,649.5	6,171.6
Earnings before Income Taxes	1,183.0	1,061.9	2,347.3	2,070.4
PEO Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,663.3	1,546.1	3,237.8	3,015.7
Earnings before Income Taxes	$ 251.7	$ 255.0	$ 477.3	$ 477.9