Stock-Based Compensation (Details) - USD ($)	12 Months Ended
	Apr. 30, 2016	Apr. 30, 2015	Apr. 30, 2014
Stock-Based Compensation
Stock based compensation expense	$ 4,562,000	$ 3,768,000	$ 3,622,000
Options
Non-vested at beginning of year (in shares)	293,907
Non-vested at end of year (in shares)		293,907
Weighted Average Grant Date Fair value
Non-vested at beginning of year (in dollars per share)	$ 10.43
Non-vested at end of year (in dollars per share)		$ 10.43
Unrecognized compensation cost related to non-vested stock awards	$ 9,872,000
Period over which unrecognized compensation cost is expected to be recognized	5 years
Weighted average period over which unrecognized compensation cost is expected to be recognized	3 years
Tax benefit realized from stock-based compensation	$ 98,000	$ 191,000	$ 2,953,000
Stock options
Weighted average assumptions used to estimate fair value of stock options granted
Expected term (in years)	6 years	6 years	6 years 29 days
Expected volatility (as a percent)	36.14%	44.65%	45.61%
Risk-free interest rate (as a percent)	1.88%	1.92%	1.64%
Weighted average fair value at grant date (in dollars per share)	$ 10.18	$ 14.05	$ 10.61
Shares
Options granted (in shares)	128,000
Intrinsic value of options
Intrinsic value of options exercised	$ 1,218,000	$ 455,000	$ 9,220,000
Intrinsic value of options outstanding	6,060,000	5,349,000
Intrinsic value of exercisable options	$ 5,047,000	$ 4,560,000
Options
Granted (in shares)	128,000
Canceled (in shares)	(37,577)
Vested (in shares)	(91,817)
Non-vested at end of year (in shares)	292,513
Weighted Average Grant Date Fair value
Granted (in dollars per share)	$ 10.18	$ 14.05	$ 10.61
Canceled (in dollars per share)	10.50
Vested (in dollars per share)	9.75
Non-vested at end of year (in dollars per share)	$ 10.53
Fair value of shares vested	$ 2,965,000	$ 2,389,000	$ 2,168,000
Proceeds from exercises of options	$ 1,122,000	$ 722,000	$ 6,709,000
Restated 2006 Plan
Stock-Based Compensation
Number of shares authorized to be issued pursuant to awards	4,884,157
Maximum number of shares that may be granted to one participant during any twelve month period	2,000,000
Maximum amount that may be paid in cash as a performance-based award to one participant during any twelve month period	$ 5,000,000
Restated 2006 Plan | Stock options
Shares
Outstanding at the beginning of the year (in shares)	657,117	713,110	892,210
Options granted (in shares)	128,000	85,599	125,000
Options exercised (in shares)	(43,000)	(30,000)	(261,900)
Options canceled (in shares)	(58,318)	(111,592)	(42,200)
Outstanding at the end of the year (in shares)	683,799	657,117	713,110
Options exercisable (in shares)	391,286
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 23.96	$ 23.20	$ 23.67
Option granted (in dollars per share)	26.83	31.27	23.39
Options exercised (in dollars per share)	21.81	23.81	24.45
Options canceled (in dollars per share)	25.98	24.75	26.05
Outstanding at the end of the year (in dollars per share)	24.46	$ 23.96	$ 23.20
Options exercisable (in dollars per share)	$ 23.48
Options
Granted (in shares)	128,000	85,599	125,000
Restated 2006 Plan | Incentive stock options | Maximum
Stock-Based Compensation
Percentage of the fair market value on date of grant	100.00%
2002 Plan | Stock options
Stock-Based Compensation
Exercisable period from grant date	5 years
Number of awards that may be granted	0
Shares
Outstanding at the beginning of the year (in shares)	44,993	48,511	177,389
Options granted (in shares)	8
Options exercised (in shares)	(31,161)	(3,518)	(121,841)
Options canceled (in shares)			(7,037)
Outstanding at the end of the year (in shares)	13,824	44,993	48,511
Options exercisable (in shares)	13,824
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 7.57	$ 7.18	$ 3.98
Option granted (in dollars per share)	2.13
Options exercised (in dollars per share)	5.70	2.13	2.25
Options canceled (in dollars per share)			11.79
Outstanding at the end of the year (in dollars per share)	11.79	$ 7.57	$ 7.18
Options exercisable (in dollars per share)	$ 11.79
Options
Granted (in shares)	8
1992 Plan | Stock options
Stock-Based Compensation
Exercisable period from grant date	5 years
Shares
Outstanding at the beginning of the year (in shares)	103,579	105,079	181,569
Options exercised (in shares)	(10,720)	(1,500)	(76,490)
Outstanding at the end of the year (in shares)	92,859	103,579	105,079
Options exercisable (in shares)	92,859
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 0.59	$ 0.52	$ 0.52
Options exercised (in dollars per share)	0.59	0.59	0.42
Outstanding at the end of the year (in dollars per share)	0.59	$ 0.59	$ 0.52
Options exercisable (in dollars per share)	$ 0.59