Consolidated Statements of Stockholders' Equity - USD ($) shares in Millions, $ in Millions	Common Stock [Member]	Additional Paid-In Capital [Member]	Retained Earnings [Member]	[1]	Accumulated Other Comprehensive Income/(Loss) [Member]	Total
Beginning balance, Shares at May. 31, 2016	360.4
Beginning balance at May. 31, 2016	$ 3.6	$ 952.7	$ 926.2		$ 29.2	$ 1,911.7
Net income			826.3			826.3	[2],[3],[4]
Unrealized gains/(losses) on securities, net of tax					(9.2)	(9.2)
Cash dividends declared			(662.3)			(662.3)
Repurchases of common shares, Shares	(2.9)
Repurchases of common shares		(5.4)	(160.8)			(166.2)
Stock-based compensation		35.4				35.4
Foreign currency translation adjustment
Activity related to equity-based plans, Shares	1.9
Activity related to equity-based plans		47.3	(18.7)			28.6
Ending balance, Shares at May. 31, 2017	359.4
Ending balance at May. 31, 2017	$ 3.6	1,030.0	1,173.6		20.0	2,227.2
Cumulative impact of adoption of ASC Topic 606			262.9			262.9
Net income			994.1			994.1	[3],[4],[5]
Unrealized gains/(losses) on securities, net of tax					(47.6)	(47.6)
Cash dividends declared			(739.7)			(739.7)
Repurchases of common shares, Shares	(2.5)
Repurchases of common shares		(4.5)	(138.6)			(143.1)
Acquisition of businesses, Shares	0.6
Acquisition of businesses		33.2				33.2
Stock-based compensation		44.6				44.6
Foreign currency translation adjustment					(8.6)	(8.6)
Activity related to equity-based plans, Shares	1.5
Activity related to equity-based plans		23.5	(26.8)			$ (3.3)
Ending balance, Shares at May. 31, 2018	359.0					359.0
Ending balance at May. 31, 2018	$ 3.6	1,126.8	1,262.6		(36.2)	$ 2,356.8	[6]
Net income			1,034.4			1,034.4
Unrealized gains/(losses) on securities, net of tax					44.0	44.0
Cash dividends declared			(826.8)			(826.8)
Repurchases of common shares, Shares	(0.7)
Repurchases of common shares		(1.4)	(55.5)			(56.9)
Stock-based compensation		46.2				46.2
Foreign currency translation adjustment					(7.7)	(7.7)
Activity related to equity-based plans, Shares	1.0
Activity related to equity-based plans		34.7	(5.2)			$ 29.5
Ending balance, Shares at May. 31, 2019	359.3					359.3
Ending balance at May. 31, 2019	$ 3.6	$ 1,206.3	$ 1,409.5		$ 0.1	$ 2,619.5

[1] Amounts have been adjusted to reflect the adoption of ASC Topic 606.

[2] Amounts have been adjusted to reflect the adoption of ASC Topic 606.

[3] Amounts have been adjusted to reflect the adoption of Accounting Standards Codification ("ASC") Topic 606, "Revenue from Contracts with Customers" ("ASC Topic 606").

[4] Amounts have been adjusted to reflect the adoption of Accounting Standards Update ("ASU") No. 2016-18, "Statement of Cash Flows (Topic 230): Restricted Cash (a consensus of the FASB Emerging Issues Task Force)."

[5] Amounts have been adjusted to reflect the adoption of ASC Topic 606.

[6] Amounts have been adjusted to reflect the adoption of ASC Topic 606 and to provide further detail related to certain professional employer organization ("PEO") balances.