Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Retained Earnings
Beginning Balance (shares) at Apr. 30, 2013		38,352,509
Beginning Balance at Apr. 30, 2013	$ 593,387	$ 23,119	$ 570,268
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	126,820		126,820
Dividends declared	(27,702)		(27,702)
Exercise of stock options (shares)		140,785
Exercise of stock options	3,368	$ 3,368
Tax benefits related to nonqualified stock options	1,791	$ 1,791
Stock-based compensation (shares)		14,093
Stock-based compensation	5,600	$ 5,600
Beginning Balance (shares) at Apr. 30, 2014		38,507,387
Beginning Balance at Apr. 30, 2014	703,264	$ 33,878	669,386
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	180,628		180,628
Dividends declared	(31,059)		(31,059)
Exercise of stock options (shares)		310,224
Exercise of stock options	11,465	$ 11,465
Tax benefits related to nonqualified stock options	3,624	$ 3,624
Stock-based compensation (shares)		68,554
Stock-based compensation	$ 7,307	$ 7,307
Beginning Balance (shares) at Apr. 30, 2015	38,886,165	38,886,165
Beginning Balance at Apr. 30, 2015	$ 875,229	$ 56,274	818,955
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	225,982		225,982
Dividends declared	(34,342)		(34,342)
Exercise of stock options (shares)		108,100
Exercise of stock options	3,717	$ 3,717
Issuance of common stock (shares)		32,717
Issuance of common stock	2,762	$ 2,762
Tax benefits related to nonqualified stock options	2,702	$ 2,702
Stock-based compensation (shares)		28,588
Stock-based compensation	$ 7,413	$ 7,413
Beginning Balance (shares) at Apr. 30, 2016	39,055,570	39,055,570
Beginning Balance at Apr. 30, 2016	$ 1,083,463	$ 72,868	$ 1,010,595