Description of Business, Basis of Presentation, and Significant Accounting Policies (Narrative) (Details) $ / shares in Units, $ in Millions		12 Months Ended
	Jul. 13, 2017 $ / shares	May 31, 2017 USD ($) segment $ / shares	May 31, 2016 USD ($)	May 31, 2015 USD ($)
Accounting Policies [Line Items]
Number of reportable segments | segment		1
Maximum period in which payroll taxes will be remitted after receipt of collections from clients		90 days
Quarterly dividend per share | $ / shares		$ 0.46
Allowance for doubtful accounts		$ 6.0	$ 4.2
Accounts receivable, net of allowance for doubtful accounts		507.5	408.6
Goodwill		657.1	657.1
Goodwill impairment loss		0.0	0.0	$ 0.0
Direct pass-through costs		5,900.0	5,100.0	4,200.0
Maximum individual workers' compensation claims liability		1.3	1.3
Workers' compensation insurance costs in current liabilities		18.0	13.8
Workers' compensation insurance costs in long-term liabilities		22.3	18.4
Maximum individual health insurance claims liability		0.3	0.3
PEO accrued health insurance reserves in current liabilities		15.7	9.5
Stock-based compensation costs recognized		$ 35.4	34.6	$ 31.4
Decrease in effective income tax rate		(1.50%)
Reserve for uncertain tax positions		$ 39.2	54.2
Excess tax benefit in income tax expense		$ 18.3
Diluted earnings per share increase | $ / shares		$ 0.05
Subsequent Event [Member]
Accounting Policies [Line Items]
Declared date	Jul. 13, 2017
Quarterly dividend per share increase | $ / shares	$ 0.04
Quarterly dividend percentage increase	9.00%
Quarterly dividend per share | $ / shares	$ 0.50
Payable date	Aug. 24, 2017
Record date	Aug. 01, 2017
Trade Receivables [Member]
Accounting Policies [Line Items]
Accounts receivable, net of allowance for doubtful accounts		$ 255.0	221.6
Purchased Receivables [Member]
Accounting Policies [Line Items]
Accounts receivable, net of allowance for doubtful accounts		$ 252.5	$ 187.0
Software Development [Member]
Accounting Policies [Line Items]
Estimated useful lives		15 years
Minimum [Member]
Accounting Policies [Line Items]
Period in which payroll taxes are typically remitted after receipt of collections from clients		1 day
Payroll funding fees average collection period		35 days
Minimum [Member] | Finite-Lived Intangible Assets [Member]
Accounting Policies [Line Items]
Amortization period		3 years
Minimum [Member] | Software [Member]
Accounting Policies [Line Items]
Estimated useful lives		3 years
Maximum [Member]
Accounting Policies [Line Items]
Period in which payroll taxes are typically remitted after receipt of collections from clients		30 days
Payroll funding fees average collection period		45 days
Short-term investments maturity at acquisition		90 days
Maximum [Member] | Finite-Lived Intangible Assets [Member]
Accounting Policies [Line Items]
Amortization period		12 years
Maximum [Member] | Software [Member]
Accounting Policies [Line Items]
Estimated useful lives		5 years
Germany [Member] | Total Revenue [Member] | Geographic Concentration Risk [Member]
Accounting Policies [Line Items]
Percentage of total revenue less than one percent		1.00%	1.00%	1.00%