Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock	Retained Earnings
Beginning Balance (shares) at Apr. 30, 2012		38,140,309
Beginning Balance at Apr. 30, 2012	$ 503,944	$ 12,199	$ 491,745
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	103,814		103,814
Dividends declared	(25,291)		(25,291)
Exercise of stock options (shares)		198,200
Exercise of stock options	4,721	$ 4,721
Tax benefits related to nonqualified stock options	1,929	$ 1,929
Stock-based compensation (shares)		14,000
Stock-based compensation	4,270	$ 4,270
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	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