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, 2015	Jan. 31, 2015	Oct. 31, 2014	Jul. 31, 2014	Apr. 30, 2014	Jan. 31, 2014	Oct. 31, 2013	Jul. 31, 2013	Apr. 30, 2015	Apr. 30, 2014	Apr. 30, 2013
Basic
Net income	$ 41,343	$ 39,319	$ 49,869	$ 50,097	$ 20,932	$ 12,663	$ 39,430	$ 53,795	$ 180,628	$ 126,820	$ 103,814
Weighted average shares outstanding-basic									38,743,227	38,457,680	38,297,083
Basic earnings per common share (in Dollars per share)	$ 1.06	$ 1.01	$ 1.29	$ 1.30	$ 0.54	$ 0.33	$ 1.03	$ 1.40	$ 4.66	$ 3.30	$ 2.71
Diluted
Plus effect of stock options and restricted stock units									360,606	410,726	322,993
Weighted-average shares outstanding-diluted									39,103,833	38,868,406	38,620,076
Diluted earnings per common share (in Dollars per share)	$ 1.05	$ 1.01	$ 1.28	$ 1.28	$ 0.54	$ 0.33	$ 1.01	$ 1.39	$ 4.62	$ 3.26	$ 2.69