Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2025	Mar. 31, 2024
Disaggregation of Revenue [Line Items]
Revenues	$ 5,553.0	$ 5,253.8	$ 15,434.1	$ 14,434.1
Other
Disaggregation of Revenue [Line Items]
Revenues	(3.4)	(2.5)	(9.6)	(9.4)
Employer Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	3,767.9	3,590.7	10,417.4	9,762.3
PEO Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	1,788.5	1,665.6	5,026.3	4,681.2
Other | Other
Disaggregation of Revenue [Line Items]
Revenues	(3.4)	(2.5)	(9.6)	(9.4)
HCM
Disaggregation of Revenue [Line Items]
Revenues	2,392.3	2,278.8	6,549.6	6,168.3
HCM | Employer Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	2,395.1	2,280.8	6,557.1	6,174.8
HCM | PEO Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
HCM | Other | Other
Disaggregation of Revenue [Line Items]
Revenues	(2.8)	(2.0)	(7.5)	(6.5)
HRO, excluding PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	1,063.6	988.1	2,872.1	2,688.5
HRO, excluding PEO zero-margin benefits pass-throughs | Employer Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	369.0	342.2	1,050.4	980.7
HRO, excluding PEO zero-margin benefits pass-throughs | PEO Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	695.2	646.4	1,823.8	1,710.7
HRO, excluding PEO zero-margin benefits pass-throughs | Other | Other
Disaggregation of Revenue [Line Items]
Revenues	(0.6)	(0.5)	(2.1)	(2.9)
PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	1,090.0	1,016.3	3,194.4	2,963.7
PEO zero-margin benefits pass-throughs | Employer Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
PEO zero-margin benefits pass-throughs | PEO Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	1,090.0	1,016.3	3,194.4	2,963.7
PEO zero-margin benefits pass-throughs | Other | Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Global
Disaggregation of Revenue [Line Items]
Revenues	651.9	649.8	1,936.7	1,865.7
Global | Employer Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	651.9	649.8	1,936.7	1,865.7
Global | PEO Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Global | Other | Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Interest on funds held for clients
Disaggregation of Revenue [Line Items]
Revenues	355.2	320.8	881.3	747.9
Interest on funds held for clients | Employer Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	351.9	317.9	873.2	741.1
Interest on funds held for clients | PEO Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	3.3	2.9	8.1	6.8
Interest on funds held for clients | Other | Other
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0