Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2021	Dec. 31, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 4,025.4	$ 3,695.7	$ 7,857.7	$ 7,166.5
Employer Services
Disaggregation of Revenue [Line Items]
Revenues	2,671.3	2,510.6	5,242.8	4,887.4
PEO Services
Disaggregation of Revenue [Line Items]
Revenues	1,358.8	1,186.1	2,622.9	2,282.0
Other
Disaggregation of Revenue [Line Items]
Revenues	(4.7)	(1.0)	(8.0)	(2.9)
HCM
Disaggregation of Revenue [Line Items]
Revenues	1,743.8	1,630.8	3,410.4	3,167.8
HCM | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	1,745.7	1,631.2	3,414.7	3,169.8
HCM | PEO Services
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
HCM | Other
Disaggregation of Revenue [Line Items]
Revenues	(1.9)	(0.4)	(4.3)	(2.0)
HRO, excluding PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	758.7	667.8	1,438.0	1,249.1
HRO, excluding PEO zero-margin benefits pass-throughs | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	267.8	249.2	524.1	477.3
HRO, excluding PEO zero-margin benefits pass-throughs | PEO Services
Disaggregation of Revenue [Line Items]
Revenues	493.7	419.2	917.6	772.7
HRO, excluding PEO zero-margin benefits pass-throughs | Other
Disaggregation of Revenue [Line Items]
Revenues	(2.8)	(0.6)	(3.7)	(0.9)
PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	863.9	765.9	1,703.4	1,507.0
PEO zero-margin benefits pass-throughs | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
PEO zero-margin benefits pass-throughs | PEO Services
Disaggregation of Revenue [Line Items]
Revenues	863.9	765.9	1,703.4	1,507.0
PEO zero-margin benefits pass-throughs | Other
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
Global
Disaggregation of Revenue [Line Items]
Revenues	553.0	525.8	1,098.8	1,030.7
Global | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	553.0	525.8	1,098.8	1,030.7
Global | PEO Services
Disaggregation of Revenue [Line Items]
Revenues		0.0	0.0	0.0
Global | 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	106.0	105.4	207.1	211.9
Interest on funds held for clients | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	104.8	104.4	205.2	209.6
Interest on funds held for clients | PEO Services
Disaggregation of Revenue [Line Items]
Revenues	1.2	1.0	1.9	2.3
Interest on funds held for clients | Other
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0