Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Mar. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 4,047.8	$ 3,828.2	$ 11,213.0	$ 10,630.9
Employer Services
Disaggregation of Revenue [Line Items]
Revenues	2,811.7	2,719.1	7,790.6	7,507.7
PEO
Disaggregation of Revenue [Line Items]
Revenues	1,238.3	1,115.5	3,429.8	3,135.1
Other
Disaggregation of Revenue [Line Items]
Revenues	(2.2)	(6.4)	(7.4)	(11.9)
HCM
Disaggregation of Revenue [Line Items]
Revenues	1,858.3	1,771.4	5,074.8	4,886.1
HCM | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	1,859.8	1,774.7	5,079.6	4,891.1
HCM | PEO
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
HCM | Other
Disaggregation of Revenue [Line Items]
Revenues	(1.5)	(3.3)	(4.8)	(5.0)
HRO, excluding PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	742.4	678.7	1,983.6	1,855.4
HRO, excluding PEO zero-margin benefits pass-throughs | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	254.1	233.2	729.7	696.6
HRO, excluding PEO zero-margin benefits pass-throughs | PEO
Disaggregation of Revenue [Line Items]
Revenues	489.0	448.6	1,256.5	1,165.7
HRO, excluding PEO zero-margin benefits pass-throughs | Other
Disaggregation of Revenue [Line Items]
Revenues	(0.7)	(3.1)	(2.6)	(6.9)
PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	747.9	665.3	2,169.4	1,965.5
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
Disaggregation of Revenue [Line Items]
Revenues	747.9	665.3	2,169.4	1,965.5
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	540.3	545.4	1,554.8	1,508.9
Global | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	540.3	545.4	1,554.8	1,508.9
Global | PEO
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	158.9	167.4	430.4	415.0
Interest on funds held for clients | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	157.5	165.8	426.5	411.1
Interest on funds held for clients | PEO
Disaggregation of Revenue [Line Items]
Revenues	1.4	1.6	3.9	3.9
Interest on funds held for clients | Other
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0