Stock-Based Compensation Stock Option Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Shares
Options granted (in shares)	0
Stock options
Intrinsic value of options
Intrinsic value of options exercised	$ 1,801	$ 1,218	$ 455
Intrinsic value of options outstanding	3,656	6,060
Intrinsic value of exercisable options	$ 3,208	$ 5,047
Restated 2006 Plan | Stock options
Shares
Outstanding at the beginning of the year (in shares)	683,799	657,117	713,110
Options granted (in shares)		128,000	85,599
Options exercised (in shares)	(167,310)	(43,000)	(30,000)
Options canceled (in shares)	(64,865)	(58,318)	(111,592)
Outstanding at the end of the year (in shares)	451,624	683,799	657,117
Options exercisable (in shares)	311,506
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 24.46	$ 23.96	$ 23.20
Option granted (in dollars per share)		26.83	31.27
Options exercised (in dollars per share)	22.32	21.81	23.81
Options canceled (in dollars per share)	26.76	25.98	24.75
Outstanding at the end of the year (in dollars per share)	24.92	$ 24.46	$ 23.96
Options exercisable (in dollars per share)	$ 24.52
2002 Plan | Stock options
Shares
Outstanding at the beginning of the year (in shares)	13,824	44,993	48,511
Options exercised (in shares)	(9,601)	(31,161)	(3,518)
Options canceled (in shares)	(4,223)	(8)
Outstanding at the end of the year (in shares)		13,824	44,993
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 11.79	$ 7.57	$ 7.18
Options exercised (in dollars per share)	11.79	5.70	2.13
Options canceled (in dollars per share)	$ 11.79	2.13
Outstanding at the end of the year (in dollars per share)		$ 11.79	$ 7.57
1992 Plan | Stock options
Shares
Outstanding at the beginning of the year (in shares)	92,859	103,579	105,079
Options exercised (in shares)	(27,219)	(10,720)	(1,500)
Outstanding at the end of the year (in shares)	65,640	92,859	103,579
Options exercisable (in shares)	65,640
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 0.59	$ 0.59	$ 0.52
Options exercised (in dollars per share)	0.59	0.59	0.59
Outstanding at the end of the year (in dollars per share)	0.59	$ 0.59	$ 0.59
Options exercisable (in dollars per share)	$ 0.59