Revenue - Disaggregation of Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 3,669.5	$ 3,492.4	$ 7,165.2	$ 6,802.7
Employer Services
Disaggregation of Revenue [Line Items]
Revenues	2,537.5	2,450.4	4,978.9	4,788.6
PEO
Disaggregation of Revenue [Line Items]
Revenues	1,134.7	1,044.7	2,191.5	2,019.6
Other
Disaggregation of Revenue [Line Items]
Revenues	(2.7)	(2.7)	(5.2)	(5.5)
HCM
Disaggregation of Revenue [Line Items]
Revenues	1,647.9	1,599.4	3,216.5	3,114.7
HCM | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	1,649.8	1,599.6	3,219.8	3,116.4
HCM | PEO
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
HCM | Other
Disaggregation of Revenue [Line Items]
Revenues	(1.9)	(0.2)	(3.3)	(1.7)
HRO, excluding PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	650.1	614.2	1,241.2	1,176.7
HRO, excluding PEO zero-margin benefits pass-throughs | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	239.9	232.9	475.6	463.4
HRO, excluding PEO zero-margin benefits pass-throughs | PEO
Disaggregation of Revenue [Line Items]
Revenues	411.0	383.8	767.5	717.1
HRO, excluding PEO zero-margin benefits pass-throughs | Other
Disaggregation of Revenue [Line Items]
Revenues	(0.8)	(2.5)	(1.9)	(3.8)
PEO zero-margin benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	722.4	659.7	1,421.5	1,300.2
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	722.4	659.7	1,421.5	1,300.2
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	511.4	490.0	1,014.5	963.5
Global | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	511.4	490.0	1,014.5	963.5
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	137.7	129.1	271.5	247.6
Interest on funds held for clients | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	136.4	127.9	269.0	245.3
Interest on funds held for clients | PEO
Disaggregation of Revenue [Line Items]
Revenues	1.3	1.2	2.5	2.3
Interest on funds held for clients | Other
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0