Financial Data by Segment and Geographic Area - Revenues, Significant Segment Expenses and Earnings before Income Taxes by Reportable Segments (Details) - USD ($) $ in Millions	12 Months Ended
	Jun. 30, 2026	Jun. 30, 2025	Jun. 30, 2024
Segment Reporting [Line Items]
Revenues	$ 21,947.4	$ 20,560.9	$ 19,202.6
Less segment expenses:
Total consolidated earnings before income taxes	5,730.3	5,310.1	4,872.3
Operating Segments
Segment Reporting [Line Items]
Revenues	21,959.5	20,573.5	19,214.4
Less segment expenses:
Total consolidated earnings before income taxes	6,372.9	5,959.0	5,477.0
Intercompany eliminations
Segment Reporting [Line Items]
Revenues	(12.1)	(12.6)	(11.8)
Operating Segments, Excluding Intersegment Elimination
Segment Reporting [Line Items]
Revenues	21,959.5	20,573.5	19,214.4
Other
Less segment expenses:
Total consolidated earnings before income taxes	(642.6)	(648.9)	(604.7)
Employer Services | Operating Segments
Segment Reporting [Line Items]
Revenues	14,831.4	13,883.1	12,980.8
Less segment expenses:
Selling and marketing	2,365.7	2,177.1	2,025.0
Zero-margin benefits pass-through costs	0.0	0.0	0.0
Workers' compensation coverage and state unemployment taxes	0.0	0.0	0.0
Other segment expenses	7,028.9	6,697.5	6,400.3
Total consolidated earnings before income taxes	5,436.8	5,008.5	4,555.5
Employer Services | Intercompany eliminations
Segment Reporting [Line Items]
Revenues	(12.1)	(12.6)	(11.8)
Employer Services | Operating Segments, Excluding Intersegment Elimination
Segment Reporting [Line Items]
Revenues	14,831.4	13,883.1	12,980.8
PEO Services | Operating Segments
Segment Reporting [Line Items]
Revenues	7,128.1	6,690.4	6,233.6
Less segment expenses:
Selling and marketing	415.6	362.8	330.5
Zero-margin benefits pass-through costs	4,607.3	4,289.0	3,975.9
Workers' compensation coverage and state unemployment taxes	718.4	681.3	613.4
Other segment expenses	450.7	406.8	392.3
Total consolidated earnings before income taxes	936.1	950.5	921.5
PEO Services | Intercompany eliminations
Segment Reporting [Line Items]
Revenues	0.0	0.0	0.0
PEO Services | Operating Segments, Excluding Intersegment Elimination
Segment Reporting [Line Items]
Revenues	7,128.1	6,690.4	6,233.6
Revenues from external customers | Operating Segments
Segment Reporting [Line Items]
Revenues	20,592.6	19,371.8	18,177.9
Revenues from external customers | Employer Services | Operating Segments
Segment Reporting [Line Items]
Revenues	13,476.9	12,692.2	11,953.6
Revenues from external customers | PEO Services | Operating Segments
Segment Reporting [Line Items]
Revenues	7,115.7	6,679.6	6,224.3
Interest on funds held for clients
Segment Reporting [Line Items]
Revenues	1,354.8	1,189.1	1,024.7
Interest on funds held for clients | Operating Segments
Segment Reporting [Line Items]
Revenues	1,354.8	1,189.1	1,024.7
Interest on funds held for clients | Employer Services | Operating Segments
Segment Reporting [Line Items]
Revenues	1,342.4	1,178.3	1,015.4
Interest on funds held for clients | PEO Services | Operating Segments
Segment Reporting [Line Items]
Revenues	$ 12.4	$ 10.8	$ 9.3