Consolidated Statements of Income and Comprehensive Income - Scenario, Unspecified [Domain] - USD ($) shares in Millions, $ in Millions	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013
Revenue:
Service revenue	$ 2,697.5	$ 2,478.2	$ 2,285.2
Interest on funds held for clients	42.1	40.7	41.0
Total revenue	2,739.6	2,518.9	2,326.2
Expenses:
Operating expenses	808.0	732.5	671.3
Selling, general and administrative expenses	878.0	803.7	750.1
Total expenses	1,686.0	1,536.2	1,421.4
Operating income	1,053.6	982.7	904.8
Investment income, net	6.4	5.4	6.6
Income before income taxes	1,060.0	988.1	911.4
Income taxes	385.1	360.6	342.4
Net income	674.9	627.5	569.0
Other comprehensive loss, net of tax:
Unrealized losses on securities, net of tax	(14.0)	(0.5)	(15.7)
Total other comprehensive loss, net of tax	(14.0)	(0.5)	(15.7)
Comprehensive income	$ 660.9	$ 627.0	$ 553.3
Basic earnings per share	$ 1.86	$ 1.72	$ 1.56
Diluted earnings per share	$ 1.85	$ 1.71	$ 1.56
Weighted-average common shares outstanding	362.9	364.5	363.8
Weighted-average common shares outstanding, assuming dilution	364.6	366.1	364.7
Cash dividends per common share	$ 1.52	$ 1.40	$ 1.31