Summary of Significant Accounting Policies (Details) - USD ($) $ / shares in Units, shares in Millions		3 Months Ended												12 Months Ended
		Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2018	Jul. 01, 2019	Jun. 30, 2017
Summary of Significant Accounting Policies [Line Items]
Net Income (Loss) Attributable to Parent		$ 411,500,000	$ 820,900,000	$ 651,600,000	$ 582,400,000	$ 475,500,000		$ 753,700,000		$ 558,200,000		$ 505,400,000		$ 2,466,500,000	$ 2,292,800,000	$ 1,884,900,000
Provision for income taxes														$ 716,100,000	$ 712,800,000	$ 397,700,000
Percent of Level Two Investment Pricing Inputs Provided by Independent Pricing Service		99.00%												99.00%
Basic weighted average shares outstanding (in shares)														430.8	435.0	440.6
Effect of Employee Stock Option Shares														0.9	1.0	1.1
Effect of Employee Restricted Stock Shares														1.0	1.6	1.6
Diluted weighted average shares outstanding (in shares)														432.7	437.6	443.3
EPS from continuing operations, Basic (in dollars per share)		$ 0.96	$ 1.91	$ 1.51	$ 1.35	$ 1.10		$ 1.74		$ 1.28		$ 1.16		$ 5.73	$ 5.27	$ 4.28
EPS from continuing operations, Diluted (in dollars per share)		$ 0.96	$ 1.90	$ 1.50	$ 1.34	$ 1.09		$ 1.73		$ 1.27		$ 1.15		$ 5.70	$ 5.24	$ 4.25
Options excluded from the calculation of diluted earnings per share because their exercise prices exceeded the average market price (in shares)														1.2	0.7	0.9
Unrecognized Tax Benefits		$ 62,300,000				$ 54,200,000								$ 62,300,000	$ 54,200,000	$ 45,200,000		$ 74,600,000
Threshold of coverage for all losses per occurrence covered by reinsurance arrangement, next fiscal year														1,000,000
Concentration Risk, Credit Risk, Financial Instruments, Off-balance Sheet Risk														242,000,000
Payments for Reinsurance														240,000,000
Tax Settlements Future Impact Potential On Earnings Maximum														4,000,000
Debt Instrument, Face Amount		2,000,000,000.0												2,000,000,000.0
Net Cash Provided by (Used in) Operating Activities														3,026,200,000	2,688,300,000	2,515,200,000
Max Expected Cash Payment On Tax Settlements														4,000,000
Total assets		39,165,500,000				41,887,700,000								39,165,500,000	41,887,700,000	38,849,100,000
Letters of Credit Outstanding, Amount		0												0
Operating lease right-of-use asset		493,700,000				0								493,700,000	0		$ 573,300,000
Operating Lease, Liability		439,900,000												$ 439,900,000
Revenue, Capitalized Costs Fulfillment Period, Description of Timing														The expected client relationship period ranges from three to eight years.
PEO revenues	[1]													$ 4,506,500,000	4,172,500,000	3,824,300,000
TOTAL REVENUES		3,376,800,000	$ 4,047,800,000	$ 3,669,500,000	$ 3,495,700,000	3,479,300,000	[2]	$ 3,828,200,000	[2]	$ 3,492,400,000	[2]	$ 3,310,300,000	[2]	14,589,800,000	14,110,200,000	13,274,200,000
Operating expenses														7,404,100,000	7,080,900,000	6,847,500,000
Total Expenses														11,555,200,000	11,215,700,000	10,819,500,000
Earnings before income taxes		$ 531,300,000	$ 1,076,700,000	$ 835,500,000	$ 739,100,000	633,300,000		$ 984,500,000		$ 741,000,000.0		$ 646,800,000		$ 3,182,600,000	3,005,600,000	2,282,600,000
Operating lease liabilities						$ 0									0		$ 522,600,000
Previously Reported [Member]
Summary of Significant Accounting Policies [Line Items]
Net Income (Loss) Attributable to Parent															2,292,800,000	1,884,900,000
Provision for income taxes															712,800,000	397,700,000
PEO revenues															4,237,500,000	3,877,800,000
TOTAL REVENUES															14,175,200,000	13,327,700,000
Operating expenses															7,145,900,000	6,901,000,000.0
Total Expenses															11,280,700,000	10,873,000,000.0
Earnings before income taxes															3,005,600,000	2,282,600,000
Restatement Adjustment [Member]
Summary of Significant Accounting Policies [Line Items]
Net Income (Loss) Attributable to Parent															0	0
Provision for income taxes															0	0
PEO revenues															(65,000,000.0)	(53,500,000)
TOTAL REVENUES															(65,000,000.0)	(53,500,000)
Operating expenses															(65,000,000.0)	(53,500,000)
Total Expenses															(65,000,000.0)	(53,500,000)
Earnings before income taxes															$ 0	$ 0
Minimum [Member]
Summary of Significant Accounting Policies [Line Items]
Revenue, Service Agreements, Term														30 days
Internal Use Software Life of Asset														three
Minimum [Member] | Nex-Gen Platform [Member]
Summary of Significant Accounting Policies [Line Items]
Internal Use Software Life of Asset														ten years
Maximum [Member]
Summary of Significant Accounting Policies [Line Items]
Revenue, Service Agreements, Term														5 years
Internal Use Software Life of Asset														five-year
Notes due on 2020 [Member]
Summary of Significant Accounting Policies [Line Items]
Debt Instrument, Term														5 years
Notes due on 2025 [Member]
Summary of Significant Accounting Policies [Line Items]
Debt Instrument, Term														10 years
Retained Earnings [Member]
Summary of Significant Accounting Policies [Line Items]
Net Income (Loss) Attributable to Parent														$ 2,466,500,000

[1] For the years ended June 30, 2020 (“fiscal 2020”), June 30, 2019 (“fiscal 2019”), and June 30, 2018 (“fiscal 2018”), Professional Employer Organization (“PEO”) revenues are net of direct pass-through costs, primarily consisting of payroll wages and payroll taxes, of $45,826.1 million, $42,688.8 million, and $39,140.9 million, respectively.

[2] (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.