Stock-Based Compensation (Details) (USD $)	12 Months Ended
	Apr. 30, 2013	Apr. 30, 2012	Apr. 30, 2011
Stock-Based Compensation
Stock based compensation expense	$ 3,470,000	$ 3,196,000	$ 2,306,000
Weighted Average Grant Date Fair value
Unrecognized compensation cost related to non-vested stock awards	9,629,000
Expected 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	4 years
Tax benefit realized from stock-based compensation	1,490,000	1,428,000	1,238,000
Stock options
Weighted average assumptions used to estimate fair value of stock options granted
Expected term	6 years	5 years 5 months 16 days	5 years
Expected volatility (as a percent)	45.94%	26.75%	24.72%
Risk-free interest rate (as a percent)	0.92%	1.40%	2.08%
Weighted average fair value at grant date (in dollars per share)	$ 8.44	$ 8.01	$ 6.48
Shares
Options granted (in shares)	203,000
Intrinsic value of options
Intrinsic value of options exercised	4,329,000	3,610,000	2,904,000
Intrinsic value of options outstanding	6,369,000	13,561,000
Intrinsic value of exercisable options	6,149,000	13,308,000
Options
Non-vested at beginning of year (in shares)	375,262
Granted (in shares)	203,000
Vested (in shares)	(143,362)
Non-vested at end of year (in shares)	434,900	375,262
Weighted Average Grant Date Fair value
Non-vested at beginning of year (in dollars per share)	$ 7.39
Granted (in dollars per share)	$ 8.44	$ 8.01	$ 6.48
Vested (in dollars per share)	$ 7.31
Non-vested at end of year (in dollars per share)	$ 7.91	$ 7.39
Fair value of shares vested	2,477,000	1,654,000	1,111,000
Proceeds from exercises of options	289,000	565,000	619,000
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)	692,210	535,410	493,210
Options granted (in shares)	203,000	175,000	72,500
Options exercised (in shares)	(3,000)	(18,200)	(17,500)
Options canceled (in shares)			(12,800)
Outstanding at the end of the year (in shares)	892,210	692,210	535,410
Options exercisable (in shares)	457,310
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 25.01	$ 23.51	$ 23.36
Option granted (in dollars per share)	$ 19.07	$ 29.28	$ 24.91
Options exercised (in dollars per share)	$ 20.75	$ 21.88	$ 22.27
Options canceled (in dollars per share)			$ 27.25
Outstanding at the end of the year (in dollars per share)	$ 23.67	$ 25.01	$ 23.51
Options exercisable (in dollars per share)	$ 23.72
Options
Granted (in shares)	203,000	175,000	72,500
Restated 2006 Plan | Incentive stock options | Minimum
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)	324,986	368,059	405,101
Options exercised (in shares)	(147,597)	(43,073)	(35,634)
Options canceled (in shares)			(1,408)
Outstanding at the end of the year (in shares)	177,389	324,986	368,059
Options exercisable (in shares)	177,389
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 2.80	$ 2.81	$ 3.10
Options exercised (in dollars per share)	$ 1.39	$ 2.91	$ 5.67
Options canceled (in dollars per share)			$ 11.79
Outstanding at the end of the year (in dollars per share)	$ 3.98	$ 2.80	$ 2.81
Options exercisable (in dollars per share)	$ 3.98
1992 Plan | Stock options
Stock-Based Compensation
Exercisable period from grant date	5 years
Shares
Outstanding at the beginning of the year (in shares)	239,310	319,573	387,000
Options exercised (in shares)	(57,741)	(80,263)	(67,427)
Outstanding at the end of the year (in shares)	181,569	239,310	319,573
Options exercisable (in shares)	181,569
Weighted Average Exercise Price
Outstanding at the beginning of the year (in dollars per share)	$ 0.49	$ 0.49	$ 0.48
Options exercised (in dollars per share)	$ 0.38	$ 0.51	$ 0.41
Outstanding at the end of the year (in dollars per share)	$ 0.52	$ 0.49	$ 0.49
Options exercisable (in dollars per share)	$ 0.52