Revenue (Disaggregation of Revenue) (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2019	Mar. 31, 2018
Disaggregation of Revenue [Line Items]
Revenues	$ 3,847.4	$ 3,696.0	$ 10,676.5	$ 10,011.5
Employer Services
Disaggregation of Revenue [Line Items]
Revenues	2,719.1	2,628.5	7,507.7	7,105.7
PEO
Disaggregation of Revenue [Line Items]
Revenues	1,134.7	1,067.3	3,180.7	2,909.5
Other
Disaggregation of Revenue [Line Items]
Revenues	(6.4)	0.2	(11.9)	(3.7)
HCM
Disaggregation of Revenue [Line Items]
Revenues	1,771.4	1,759.4	4,886.1	4,696.4
HCM | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	1,774.7	1,756.3	4,891.1	4,694.1
HCM | PEO
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
HCM | Other
Disaggregation of Revenue [Line Items]
Revenues	(3.3)	3.1	(5.0)	2.3
HRO, excluding PEO benefits pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	678.7	661.7	1,855.4	1,713.6
HRO, excluding PEO benefits pass-throughs | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	233.2	225.1	696.6	642.2
HRO, excluding PEO benefits pass-throughs | PEO
Disaggregation of Revenue [Line Items]
Revenues	448.6	439.5	1,165.7	1,077.4
HRO, excluding PEO benefits pass-throughs | Other
Disaggregation of Revenue [Line Items]
Revenues	(3.1)	(2.9)	(6.9)	(6.0)
PEO Benefits Pass-throughs
Disaggregation of Revenue [Line Items]
Revenues	684.5	626.4	2,011.1	1,828.7
PEO Benefits Pass-throughs | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	0.0	0.0	0.0	0.0
PEO Benefits Pass-throughs | PEO
Disaggregation of Revenue [Line Items]
Revenues	684.5	626.4	2,011.1	1,828.7
PEO 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	545.4	513.7	1,508.9	1,431.9
Global | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	545.4	513.7	1,508.9	1,431.9
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
Client Fund Interest
Disaggregation of Revenue [Line Items]
Revenues	167.4	134.8	415.0	340.9
Client Fund Interest | Employer Services
Disaggregation of Revenue [Line Items]
Revenues	165.8	133.4	411.1	337.5
Client Fund Interest | PEO
Disaggregation of Revenue [Line Items]
Revenues	1.6	1.4	3.9	3.4
Client Fund Interest | Other
Disaggregation of Revenue [Line Items]
Revenues	$ 0.0	$ 0.0	$ 0.0	$ 0.0