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, 2017	Jan. 31, 2017	Oct. 31, 2016	Jul. 31, 2016	Apr. 30, 2016	Jan. 31, 2016	Oct. 31, 2015	Jul. 31, 2015	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Basic
Net income	$ 30,078	$ 22,835	$ 57,180	$ 67,392	$ 47,044	$ 38,099	$ 79,033	$ 61,806	$ 177,485	$ 225,982	$ 180,628
Weighted average shares outstanding-basic (shares)									39,124,665	39,016,299	38,743,227
Basic earnings per common share (in Dollars per share)	$ 0.77	$ 0.58	$ 1.46	$ 1.72	$ 1.20	$ 0.98	$ 2.03	$ 1.59	$ 4.54	$ 5.79	$ 4.66
Diluted
Plus effect of stock options and restricted stock units (shares)									454,333	405,900	360,606
Weighted-average shares outstanding-diluted (shares)									39,578,998	39,422,199	39,103,833
Diluted earnings per common share (in Dollars per share)	$ 0.76	$ 0.58	$ 1.44	$ 1.70	$ 1.19	$ 0.97	$ 2.00	$ 1.57	$ 4.48	$ 5.73	$ 4.62