Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Retained Earnings
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
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
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	177,485		177,485
Dividends declared	(37,534)		(37,534)
Exercise of stock options (shares)		69,150
Exercise of stock options	2,357	$ 2,357
Issuance of common stock (shares)		28,138
Issuance of common stock	$ 3,526	$ 3,526
Stock-based compensation (shares)		56,763
Repurchase of common stock (shares)	(443,800)
Repurchase of common stock	$ (49,374)	$ (49,374)
Stock-based compensation	$ 10,697	$ 10,697
Beginning Balance (shares) at Apr. 30, 2017	38,765,821	38,765,821
Beginning Balance at Apr. 30, 2017	$ 1,190,620	$ 40,074	$ 1,150,546