Income Taxes (Components of Provision for Income Taxes) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	May 31, 2017	Feb. 28, 2017	Nov. 30, 2016	Aug. 31, 2016	May 31, 2016	Feb. 29, 2016	Nov. 30, 2015	Aug. 31, 2015	May 31, 2017	May 31, 2016	May 31, 2015
Income Taxes [Abstract]
Federal									$ 362.0	$ 336.4	$ 341.4
State									48.1	50.8	47.8
Total current									410.1	387.2	389.2
Federal									15.9	5.5	(5.1)
State									1.5	1.6	1.0
Total deferred									17.4	7.1	(4.1)
Income taxes	$ 105.2	$ 105.3	$ 109.9	$ 107.1	$ 98.0	$ 101.3	$ 106.6	$ 88.4	$ 427.5	$ 394.3	$ 385.1