Stock-Based Compensation Stock Option Plans (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Shares
Options granted (in shares)	0	0
Stock options
Intrinsic value of options
Intrinsic value of options exercised	$ 2,407	$ 1,747	$ 1,198
Intrinsic value of options outstanding	10,890	2,840
Intrinsic value of exercisable options	$ 8,587	$ 2,454
Restated 2006 Plan | Stock options
Shares
Outstanding at the beginning of the year (in shares)	446,624	678,799	652,117
Options granted (in shares)			128,000
Options exercised (in shares)	(107,598)	(167,310)	(43,000)
Options canceled (in shares)		(64,865)	(58,318)
Outstanding at the end of the year (in shares)	339,026	446,624	678,799
Options exercisable (in shares)	256,012
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 24.93	$ 24.46	$ 23.96
Option granted (in dollars per share)			26.83
Options exercised (in dollars per share)	23.80	22.32	21.81
Options canceled (in dollars per share)		26.76	25.98
Outstanding at the end of the year (in dollars per share)	25.29	$ 24.93	$ 24.46
Options exercisable (in dollars per share)	$ 24.81
2002 Plan | Stock options
Shares
Outstanding at the beginning of the year (in shares)		13,824	44,993
Options exercised (in shares)		(9,601)	(31,161)
Options canceled (in shares)		(4,223)	(8)
Outstanding at the end of the year (in shares)			13,824
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)		$ 11.79	$ 7.57
Options exercised (in dollars per share)		11.79	5.70
Options canceled (in dollars per share)		$ 11.79	2.13
Outstanding at the end of the year (in dollars per share)			$ 11.79
1992 Plan | Stock options
Shares
Outstanding at the beginning of the year (in shares)	43,415	68,604	78,604
Options exercised (in shares)	(25,113)	(25,189)	(10,000)
Outstanding at the end of the year (in shares)	18,302	43,415	68,604
Options exercisable (in shares)	18,302
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 0.59	$ 0.59	$ 0.59
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