Stock-Based Compensation - Stock Options Granted and Outstanding (Details) - $ / shares	12 Months Ended
	May 31, 2018	May 31, 2017	May 31, 2016	May 31, 2015
Shares
Outstanding, beginning of period (shares)	8,588,050	8,419,907	7,835,570
Granted (shares)	1,664,867	1,343,180	1,739,767
Canceled (shares)	(7,809)	(5,368)	(4,413)
Forfeited (shares)	(255,627)	(165,452)	(231,042)
Exercised (shares)	(1,059,295)	(1,004,217)	(919,975)
Outstanding, end of period (shares)	8,930,186	8,588,050	8,419,907
Weighted Average Exercise Price
Outstanding, beginning of period, Weighted Average Exercise Price (dollars per share)	$ 74.77	$ 61.83	$ 51.59
Granted, Weighted Average Exercise Price (dollars per share)	175.86	126.51	93.55
Canceled, Weighted Average Exercise Price (dollars per share)	45.10	32.45	34.56
Forfeited, Weighed Average Exercise Price (dollars per share)	94.73	73.43	60.57
Exercised, Weighted Average Exercise Price (dollars per share)	44.06	35.95	35.07
Outstanding, end of period, Weighted Average Exercise Price (dollars per share)	$ 96.71	$ 74.77	$ 61.83
Number of exercisable options (shares)	2,006,922	1,795,898	1,649,236	1,426,550