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, 2018	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2017	Jan. 31, 2017	Oct. 31, 2016	Jul. 31, 2016	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Basic
Net income	$ 19,262	$ 192,965	$ 48,918	$ 56,758	$ 30,078	$ 22,835	$ 57,180	$ 67,392	$ 317,903	$ 177,485	$ 225,982
Weighted average shares outstanding-basic (shares)									37,778,304	39,124,665	39,016,299
Basic earnings per common share (in Dollars per share)	$ 0.52	$ 5.13	$ 1.29	$ 1.48	$ 0.77	$ 0.58	$ 1.46	$ 1.72	$ 8.41	$ 4.54	$ 5.79
Diluted
Plus effect of stock options and restricted stock units (shares)									353,795	454,333	405,900
Weighted-average shares outstanding-diluted (shares)									38,132,099	39,578,998	39,422,199
Diluted earnings per common share (in Dollars per share)	$ 0.51	$ 5.08	$ 1.28	$ 1.46	$ 0.76	$ 0.58	$ 1.44	$ 1.70	$ 8.34	$ 4.48	$ 5.73