Quarterly Financial Data (Unaudited) - Additional Disclosures (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	May 31, 2014	Feb. 28, 2014	Nov. 30, 2013	Aug. 31, 2013	May 31, 2013	Feb. 28, 2013	Nov. 30, 2012	Aug. 31, 2012	May 31, 2014	May 31, 2013	May 31, 2012
Service revenue	$ 628.8	$ 639.9	$ 606.4	$ 603.1	$ 575.3	$ 582.4	$ 559.4	$ 568.1	$ 2,478.2	$ 2,285.2	$ 2,186.2
Total revenue	639.0	650.4	616.4	613.1	585.3	593.3	569.4	578.2	2,518.9	2,326.2	2,229.8
Scenario, Previously Reported [Member]
Service revenue		626.0	600.5	597.9
Total revenue		$ 636.5	$ 610.5	$ 607.9
Settlement of State Income Tax Matter [Member]
Impact to diluted earnings per share from settlement of state tax item										$ (0.04)