Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2023	Dec. 31, 2022
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	$ 4,668.0	$ 4,391.0	$ 9,180.3	$ 8,606.6
Employer Services
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	3,125.2	2,891.9	6,171.6	5,682.2
PEO Services
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	1,546.1	1,503.3	3,015.7	2,932.7
Other
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	(3.3)	(4.2)	(7.0)	(8.3)
HCM
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	1,975.2	1,861.8	3,889.4	3,672.8
HCM | Employer Services
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	1,977.4	1,864.1	3,894.0	3,677.4
HCM | PEO Services
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	0.0	0.0	0.0	0.0
HCM | Other
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	(2.2)	(2.3)	(4.6)	(4.6)
HRO, excluding PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	888.7	853.9	1,700.4	1,631.2
HRO, excluding PEO zero-margin benefits pass-throughs | Employer Services
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	316.4	298.4	638.5	595.2
HRO, excluding PEO zero-margin benefits pass-throughs | PEO Services
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	573.4	557.4	1,064.3	1,039.7
HRO, excluding PEO zero-margin benefits pass-throughs | Other
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	(1.1)	(1.9)	(2.4)	(3.7)
PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	970.7	944.2	1,947.5	1,890.0
PEO zero-margin benefits pass-throughs | Employer Services
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	0.0		0.0	0.0
PEO zero-margin benefits pass-throughs | PEO Services
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	970.7	944.2	1,947.5	1,890.0
PEO zero-margin benefits pass-throughs | Other
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	0.0	0.0	0.0	0.0
Global
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	608.1	543.9	1,215.9	1,084.3
Global | Employer Services
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	608.1	543.9	1,215.9	1,084.3
Global | PEO Services
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	0.0	0.0	0.0	0.0
Global | Other
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	0.0	0.0	0.0	0.0
Interest on funds held for clients
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	225.3	187.2	427.1	328.3
Interest on funds held for clients | Employer Services
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	223.3	185.5	423.2	325.3
Interest on funds held for clients | PEO Services
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	2.0	1.7	3.9	3.0
Interest on funds held for clients | Other
Disaggregation of Revenue [Line Items]
TOTAL REVENUES	$ 0.0	$ 0.0	$ 0.0	$ 0.0