Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2024	Dec. 31, 2023
Disaggregation of Revenue [Line Items]
Revenues	$ 5,048.4	$ 4,668.0	$ 9,881.1	$ 9,180.3
Other
Disaggregation of Revenue [Line Items]
Revenues	(3.4)	(3.3)	(6.2)	(7.0)
Employer Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	3,388.5	3,125.2	6,649.5	6,171.6
PEO Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	1,663.3	1,546.1	3,237.8	3,015.7
Other | Other
Disaggregation of Revenue [Line Items]
Revenues	(3.4)	(3.3)	(6.2)	(7.0)
HCM
Disaggregation of Revenue [Line Items]
Revenues	2,131.9	1,975.2	4,157.3	3,889.4
HCM | Employer Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	2,134.4	1,977.4	4,162.0	3,894.0
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	(2.5)	(2.2)	(4.7)	(4.6)
HRO, excluding PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	943.2	888.7	1,808.4	1,700.4
HRO, excluding PEO zero-margin benefits pass-throughs | Employer Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	338.5	316.4	681.3	638.5
HRO, excluding PEO zero-margin benefits pass-throughs | PEO Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	605.6	573.4	1,128.6	1,064.3
HRO, excluding PEO zero-margin benefits pass-throughs | Other | Other
Disaggregation of Revenue [Line Items]
Revenues	(0.9)	(1.1)	(1.5)	(2.4)
PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	1,055.2	970.7	2,104.4	1,947.5
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	1,055.2	970.7	2,104.4	1,947.5
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	645.3	608.1	1,284.9	1,215.9
Global | Employer Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	645.3	608.1	1,284.9	1,215.9
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	272.8	225.3	526.1	427.1
Interest on funds held for clients | Employer Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	270.3	223.3	521.3	423.2
Interest on funds held for clients | PEO Services | Operating Segments
Disaggregation of Revenue [Line Items]
Revenues	2.5	2.0	4.8	3.9
Interest on funds held for clients | Other | Other
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0