Stock-Based Compensation - Restricted Stock Awards Granted and Outstanding (Details) - Restricted Stock Awards - $ / shares	12 Months Ended
	May. 31, 2015	May. 31, 2014	May. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested [Roll Forward]
Outstanding, beginning of period (shares)	2,158,778	2,015,023	1,888,996
Granted (shares)	627,033	661,514	810,453
Canceled (shares)	(50,277)	(52,124)	(73,856)
Vested (shares)	(525,421)	(465,635)	(610,570)
Outstanding, end of period (shares)	2,210,113	2,158,778	2,015,023
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Roll Forward]
Outstanding, Weighted Average Grant Price, beginning of period (dollars per share)	$ 45.04	$ 35.97	$ 29.93
Granted, Weighted Average Grant Price (dollars per share)	80.73	60.66	41.72
Canceled, Weighted Average Grant Price (dollars per share)	49.33	37.95	31.78
Vested, Weighted Average Grant Price (dollars per share)	34.39	28.76	25.40
Outstanding, Weighted Average Grant Price, end of period (dollars per share)	$ 57.60	$ 45.04	$ 35.97