Stock-Based Compensation (Details 2) (USD $)	12 Months Ended
	Apr. 30, 2013
Stock-Based Compensation
Exercise Price (in dollars per share)	$ 17.52
Range of Exercise Price, low end of range (in dollars per share)	$ 0.37
Range of Exercise Price, high end of range (in dollars per share)	$ 32.19
Options Outstanding
Number of Options (in shares)	1,251,168
Weighted Average Remaining Contractual Life In Years	5 years 11 months 19 days
Weighted Average Exercise Price (in dollars per share)	$ 17.52
Options Exercisable
Number of Options (in shares)	816,268
Weighted Average Exercise Price (in dollars per share)	$ 14.27
Weighted Average Remaining Contractual Life	4 years 6 months
0.37
Stock-Based Compensation
Exercise Price (in dollars per share)	$ 0.37
Options Outstanding
Number of Options (in shares)	57,415
Weighted Average Remaining Contractual Life In Years	10 months 20 days
Weighted Average Exercise Price (in dollars per share)	$ 0.37
Options Exercisable
Number of Options (in shares)	57,415
Weighted Average Exercise Price (in dollars per share)	$ 0.37
0.59
Stock-Based Compensation
Exercise Price (in dollars per share)	$ 0.59
Options Outstanding
Number of Options (in shares)	124,154
Weighted Average Remaining Contractual Life In Years	6 years 4 months 6 days
Weighted Average Exercise Price (in dollars per share)	$ 0.59
Options Exercisable
Number of Options (in shares)	124,154
Weighted Average Exercise Price (in dollars per share)	$ 0.59
0.78
Stock-Based Compensation
Exercise Price (in dollars per share)	$ 0.78
Options Outstanding
Number of Options (in shares)	53,288
Weighted Average Remaining Contractual Life In Years	1 year 1 month 28 days
Weighted Average Exercise Price (in dollars per share)	$ 0.78
Options Exercisable
Number of Options (in shares)	53,288
Weighted Average Exercise Price (in dollars per share)	$ 0.78
2.13
Stock-Based Compensation
Exercise Price (in dollars per share)	$ 2.13
Options Outstanding
Number of Options (in shares)	82,766
Weighted Average Remaining Contractual Life In Years	2 years 5 months 19 days
Weighted Average Exercise Price (in dollars per share)	$ 2.13
Options Exercisable
Number of Options (in shares)	82,766
Weighted Average Exercise Price (in dollars per share)	$ 2.13
11.79
Stock-Based Compensation
Exercise Price (in dollars per share)	$ 11.79
Options Outstanding
Number of Options (in shares)	41,335
Weighted Average Remaining Contractual Life In Years	3 years 4 months 24 days
Weighted Average Exercise Price (in dollars per share)	$ 11.79
Options Exercisable
Number of Options (in shares)	41,335
Weighted Average Exercise Price (in dollars per share)	$ 11.79
18.07-24.65
Stock-Based Compensation
Exercise Price (in dollars per share)	$ 21.19
Range of Exercise Price, low end of range (in dollars per share)	$ 18.07
Range of Exercise Price, high end of range (in dollars per share)	$ 24.65
Options Outstanding
Number of Options (in shares)	620,210
Weighted Average Remaining Contractual Life In Years	6 years 7 months 10 days
Weighted Average Exercise Price (in dollars per share)	$ 21.19
Options Exercisable
Number of Options (in shares)	373,710
Weighted Average Exercise Price (in dollars per share)	$ 22.21
25.77-32.19
Stock-Based Compensation
Exercise Price (in dollars per share)	$ 29.34
Range of Exercise Price, low end of range (in dollars per share)	$ 25.77
Range of Exercise Price, high end of range (in dollars per share)	$ 32.19
Options Outstanding
Number of Options (in shares)	272,000
Weighted Average Remaining Contractual Life In Years	7 years 9 months 18 days
Weighted Average Exercise Price (in dollars per share)	$ 29.34
Options Exercisable
Number of Options (in shares)	83,600
Weighted Average Exercise Price (in dollars per share)	$ 30.43