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
	Sep. 30, 2025	Sep. 30, 2024
Segment Reporting Information [Line Items]
Revenues	$ 5,175.2	$ 4,832.7
Less segment expenses:
Total consolidated earnings before income taxes	1,307.5	1,236.2
Operating Segments
Less segment expenses:
Total consolidated earnings before income taxes	1,447.6	1,389.9
Intercompany eliminations
Segment Reporting Information [Line Items]
Revenues	(3.5)	(2.8)
Operating Segments, Excluding Intersegment Elimination
Segment Reporting Information [Line Items]
Revenues	5,178.7	4,835.5
Other
Less segment expenses:
Total consolidated earnings before income taxes	(140.1)	(153.7)
Employer Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	3,491.2	3,261.0
Less segment expenses:
Selling and marketing	552.4	505.3
Zero-margin benefits pass-through costs	0.0	0.0
Worker's compensation coverage and state unemployment taxes	0.0	0.0
Other segment expenses	1,710.0	1,591.4
Total consolidated earnings before income taxes	1,228.8	1,164.3
Employer Services | Intercompany eliminations
Segment Reporting Information [Line Items]
Revenues	(3.5)	(2.8)
Employer Services | Operating Segments, Excluding Intersegment Elimination
Segment Reporting Information [Line Items]
Revenues	3,491.2	3,261.0
PEO Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,687.5	1,574.5
Less segment expenses:
Selling and marketing	91.2	81.1
Zero-margin benefits pass-through costs	1,131.0	1,049.2
Worker's compensation coverage and state unemployment taxes	134.9	124.3
Other segment expenses	111.6	94.3
Total consolidated earnings before income taxes	218.8	225.6
PEO Services | Intercompany eliminations
Segment Reporting Information [Line Items]
Revenues	0.0	0.0
PEO Services | Operating Segments, Excluding Intersegment Elimination
Segment Reporting Information [Line Items]
Revenues	1,687.5	1,574.5
Revenues from external customers | Operating Segments
Segment Reporting Information [Line Items]
Revenues	4,888.4	4,579.4
Revenues from external customers | Employer Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	3,203.5	3,007.2
Revenues from external customers | PEO Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	1,684.9	1,572.2
Interest on funds held for clients
Segment Reporting Information [Line Items]
Revenues	286.8	253.3
Interest on funds held for clients | Operating Segments
Segment Reporting Information [Line Items]
Revenues	286.8	253.3
Interest on funds held for clients | Employer Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	284.2	251.0
Interest on funds held for clients | PEO Services | Operating Segments
Segment Reporting Information [Line Items]
Revenues	$ 2.6	$ 2.3