Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Disaggregation of Revenue [Line Items]
Revenues	$ 4,102.0	$ 4,047.8	$ 11,268.4	$ 11,213.0
Employer Services
Disaggregation of Revenue [Line Items]
Revenues	2,780.0	2,811.7	7,667.4	7,790.6
PEO Services
Disaggregation of Revenue [Line Items]
Revenues	1,324.1	1,238.3	3,606.1	3,429.8
Other
Disaggregation of Revenue [Line Items]
Revenues	(2.1)	(2.2)	(5.1)	(7.4)
HCM
Disaggregation of Revenue [Line Items]
Revenues	1,851.3	1,858.3	5,006.7	5,074.8
HCM | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	1,852.6	1,859.8	5,010.1	5,079.6
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	(1.3)	(1.5)	(3.4)	(4.8)
HRO, excluding PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	791.5	742.4	2,042.6	1,983.6
HRO, excluding PEO zero-margin benefits pass-throughs | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	253.6	254.1	732.9	729.7
HRO, excluding PEO zero-margin benefits pass-throughs | PEO Services
Disaggregation of Revenue [Line Items]
Revenues	538.7	489.0	1,311.4	1,256.5
HRO, excluding PEO zero-margin benefits pass-throughs | Other
Disaggregation of Revenue [Line Items]
Revenues	(0.8)	(0.7)	(1.7)	(2.6)
PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	784.7	747.9	2,291.7	2,169.4
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	784.7	747.9	2,291.7	2,169.4
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	567.1	540.3	1,608.2	1,554.8
Global | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	567.1	540.3	1,608.2	1,554.8
Global | PEO Services
Disaggregation of Revenue [Line Items]
Revenues	0.0	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	107.4	158.9	319.2	430.4
Interest on funds held for clients | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	106.7	157.5	316.2	426.5
Interest on funds held for clients | PEO Services
Disaggregation of Revenue [Line Items]
Revenues	0.7	1.4	3.0	3.9
Interest on funds held for clients | Other
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0