Financial Data By Segment (Financial Data By Strategic Business Unit Segment) (Details) $ in Millions	3 Months Ended												12 Months Ended
	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)		Mar. 31, 2019 USD ($)		Dec. 31, 2018 USD ($)		Sep. 30, 2018 USD ($)		Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)
Segment Reporting Information [Line Items]
TOTAL REVENUES	$ 3,376.8	$ 4,047.8	$ 3,669.5	$ 3,495.7	$ 3,479.3	[1]	$ 3,828.2	[1]	$ 3,492.4	[1]	$ 3,310.3	[1]	$ 14,589.8	$ 14,110.2	$ 13,274.2
EARNINGS FROM CONTINUING OPERATIONS BEFORE INCOME TAXES	531.3	$ 1,076.7	$ 835.5	$ 739.1	633.3		$ 984.5		$ 741.0		$ 646.8		3,182.6	3,005.6	2,282.6
Total assets	39,165.5				41,887.7								39,165.5	41,887.7	38,849.1
Capital Expenditures from continuing operations													168.3	162.7	191.9
Depreciation, Depletion and Amortization													$ 480.0	409.0	377.6
Number of Reportable Segments | segment													2
Employer Services [Member]
Segment Reporting Information [Line Items]
TOTAL REVENUES													$ 10,086.6	9,942.8	9,454.8
EARNINGS FROM CONTINUING OPERATIONS BEFORE INCOME TAXES													3,063.0	2,960.9	2,601.1
Total assets	37,071.7				34,606.3								37,071.7	34,606.3	31,984.2
Capital Expenditures from continuing operations													115.7	98.2	113.9
Depreciation, Depletion and Amortization													388.0	321.0	291.9
PEO Services [Member]
Segment Reporting Information [Line Items]
TOTAL REVENUES													4,511.5	4,177.7	3,828.8
EARNINGS FROM CONTINUING OPERATIONS BEFORE INCOME TAXES													605.5	616.2	541.6
Total assets	1,443.2				1,584.1								1,443.2	1,584.1	1,329.8
Capital Expenditures from continuing operations													0.0	0.0	0.0
Depreciation, Depletion and Amortization													3.4	3.5	3.0
Other [Member]
Segment Reporting Information [Line Items]
TOTAL REVENUES													8.3	(10.3)	(9.4)
EARNINGS FROM CONTINUING OPERATIONS BEFORE INCOME TAXES													(485.9)	(571.5)	(860.1)
Total assets	$ 650.6				$ 5,697.3								650.6	5,697.3	5,535.1
Capital Expenditures from continuing operations													52.6	64.5	78.0
Depreciation, Depletion and Amortization													$ 88.6	$ 84.5	$ 82.7

[1] (A) The prior period amounts presented have been revised to correct the amounts previously reported on a gross basis to a net basis by reducing PEO revenues and operating expenses for associated costs of an equal amount of $12.9 million, $13.5 million, $19.2 million and $19.4 million for the first quarter, second quarter, third quarter and fourth quarter, respectively. Refer to Note 1 to our consolidated financial statements for more information on this revision.