Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019
Disaggregation of Revenue [Line Items]
Revenues	$ 3,695.7	$ 3,669.5	$ 7,166.5	$ 7,165.2
Employer Services
Disaggregation of Revenue [Line Items]
Revenues	2,510.6	2,537.5	4,887.4	4,978.9
PEO Services
Disaggregation of Revenue [Line Items]
Revenues	1,186.1	1,134.7	2,282.0	2,191.5
Other
Disaggregation of Revenue [Line Items]
Revenues	(1.0)	(2.7)	(2.9)	(5.2)
HCM
Disaggregation of Revenue [Line Items]
Revenues	1,624.8	1,647.9	3,155.4	3,216.5
HCM | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	1,625.2	1,649.8	3,157.4	3,219.8
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	(0.4)	(1.9)	(2.0)	(3.3)
HRO, excluding PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	668.7	650.1	1,251.1	1,241.2
HRO, excluding PEO zero-margin benefits pass-throughs | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	250.1	239.9	479.3	475.6
HRO, excluding PEO zero-margin benefits pass-throughs | PEO Services
Disaggregation of Revenue [Line Items]
Revenues	419.2	411.0	772.7	767.5
HRO, excluding PEO zero-margin benefits pass-throughs | Other
Disaggregation of Revenue [Line Items]
Revenues	(0.6)	(0.8)	(0.9)	(1.9)
PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	765.9	722.4	1,507.0	1,421.5
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 Services
Disaggregation of Revenue [Line Items]
Revenues	765.9	722.4	1,507.0	1,421.5
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	530.9	511.4	1,041.1	1,014.5
Global | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	530.9	511.4	1,041.1	1,014.5
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	105.4	137.7	211.9	271.5
Interest on funds held for clients | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	104.4	136.4	209.6	269.0
Interest on funds held for clients | PEO Services
Disaggregation of Revenue [Line Items]
Revenues	1.0	1.3	2.3	2.5
Interest on funds held for clients | Other
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0