Interim Financial Data by Segment - Schedule of Revenues, Significant Segment Expenses and Earnings before Income Taxes by Reportable Segments (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2025	Dec. 31, 2024
Segment Reporting Information [Line Items]
Revenues	$ 5,359.3	$ 5,048.4	$ 10,534.5	$ 9,881.1
Less segment expenses:
Total consolidated earnings before income taxes	1,383.2	1,261.3	2,690.7	2,497.5
Operating Segments
Less segment expenses:
Total consolidated earnings before income taxes	1,531.7	1,434.7	2,979.3	2,824.6
Intercompany eliminations
Segment Reporting Information [Line Items]
Revenues	(3.3)	(3.4)	(6.8)	(6.2)
Operating Segments, Excluding Intersegment Elimination
Segment Reporting Information [Line Items]
Revenues	5,362.6	5,051.8	10,541.3	9,887.3
Other
Less segment expenses:
Total consolidated earnings before income taxes	(148.5)	(173.4)	(288.6)	(327.1)
Employer Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	3,607.4	3,388.5	7,098.6	6,649.5
Less segment expenses:
Selling and marketing	578.1	531.6	1,130.5	1,036.9
Zero-margin benefits pass-through costs	0.0	0.0	0.0	0.0
Worker's compensation coverage and state unemployment taxes	0.0	0.0	0.0	0.0
Other segment expenses	1,750.5	1,673.9	3,460.5	3,265.3
Total consolidated earnings before income taxes	1,278.8	1,183.0	2,507.6	2,347.3
Employer Services | Intercompany eliminations
Segment Reporting Information [Line Items]
Revenues	(3.3)	(3.4)	(6.8)	(6.2)
Employer Services | Operating Segments, Excluding Intersegment Elimination
Segment Reporting Information [Line Items]
Revenues	3,607.4	3,388.5	7,098.6	6,649.5
PEO Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,755.2	1,663.3	3,442.7	3,237.8
Less segment expenses:
Selling and marketing	104.2	91.1	195.5	172.1
Zero-margin benefits pass-through costs	1,126.2	1,055.2	2,257.2	2,104.4
Worker's compensation coverage and state unemployment taxes	158.5	156.1	293.4	280.4
Other segment expenses	113.4	109.2	224.9	203.6
Total consolidated earnings before income taxes	252.9	251.7	471.7	477.3
PEO Services | Intercompany eliminations
Segment Reporting Information [Line Items]
Revenues	0.0	0.0	0.0	0.0
PEO Services | Operating Segments, Excluding Intersegment Elimination
Segment Reporting Information [Line Items]
Revenues	1,755.2	1,663.3	3,442.7	3,237.8
Revenues from external customers | Operating Segments
Segment Reporting Information [Line Items]
Revenues	5,050.7	4,775.6	9,939.0	9,355.0
Revenues from external customers | Employer Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	3,298.3	3,114.8	6,501.7	6,122.0
Revenues from external customers | PEO Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,752.4	1,660.8	3,437.3	3,233.0
Interest on funds held for clients
Segment Reporting Information [Line Items]
Revenues	308.6	272.8	595.5	526.1
Interest on funds held for clients | Operating Segments
Segment Reporting Information [Line Items]
Revenues	308.6	272.8	595.5	526.1
Interest on funds held for clients | Employer Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	305.8	270.3	590.1	521.3
Interest on funds held for clients | PEO Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	$ 2.8	$ 2.5	$ 5.4	$ 4.8