Net Income Per Common Share - Schedule of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Apr. 30, 2019	Jan. 31, 2019	Oct. 31, 2018	Jul. 31, 2018	Apr. 30, 2018	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2019	Apr. 30, 2018	Apr. 30, 2017
Basic
Net income	$ 25,212	$ 41,835	$ 66,615	$ 70,224	$ 19,262	$ 192,965	$ 48,918	$ 56,758	$ 203,886	$ 317,903	$ 177,485
Weighted average shares outstanding-basic (shares)									36,709,940	37,778,304	39,124,665
Basic earnings per common share (in Dollars per share)	$ 0.69	$ 1.14	$ 1.82	$ 1.92	$ 0.52	$ 5.13	$ 1.29	$ 1.48	$ 5.55	$ 8.41	$ 4.54
Diluted
Plus effect of stock options and restricted stock units (shares)									265,447	353,795	454,333
Weighted-average shares outstanding-diluted (shares)									36,975,387	38,132,099	39,578,998
Diluted earnings per common share (in Dollars per share)	$ 0.68	$ 1.13	$ 1.80	$ 1.90	$ 0.51	$ 5.08	$ 1.28	$ 1.46	$ 5.51	$ 8.34	$ 4.48