Fair Value of Financial Assets and Liabilities (Details 2) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2012	Apr. 30, 2012
Cash equivalents:
Restricted cash	$ 604	$ 500
Short-term investments:
Short-term investments	74,660	50,834
Quoted prices in active markets for identical assets (Level 1) [Member]
Cash equivalents:
Restricted cash	604	500
Significant other observable inputs (Level 2) [Member]
Cash equivalents:
Restricted cash
Significant unobservable inputs (Level 3) [Member]
Cash equivalents:
Restricted cash
Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	5,447	29,006
Short-term investments:
Short-term investments	74,660	50,834
Total assets	80,711	80,340
Fair Value, Measurements, Recurring [Member] | Certificates of deposit (U.S. Gov. guaranteed) [Member]
Short-term investments:
Short-term investments	7,344	4,496
Fair Value, Measurements, Recurring [Member] | Money market funds [Member]
Cash equivalents:
Cash equivalents	447	288
Fair Value, Measurements, Recurring [Member] | U.S. Treasury bills [Member]
Cash equivalents:
Cash equivalents	5,000	7,499
Fair Value, Measurements, Recurring [Member] | Corporate bonds (U.S. Gov. guaranteed) [Member]
Cash equivalents:
Cash equivalents		21,219
Short-term investments:
Short-term investments	18,167	18,239
Fair Value, Measurements, Recurring [Member] | U.S. Treasury notes [Member]
Short-term investments:
Short-term investments	49,149	28,099
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member]
Cash equivalents:
Cash equivalents	5,447	29,006
Short-term investments:
Short-term investments	74,660	50,834
Total assets	80,711	80,340
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Certificates of deposit (U.S. Gov. guaranteed) [Member]
Short-term investments:
Short-term investments	7,344	4,496
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Money market funds [Member]
Cash equivalents:
Cash equivalents	447	288
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | U.S. Treasury bills [Member]
Cash equivalents:
Cash equivalents	5,000	7,499
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | Corporate bonds (U.S. Gov. guaranteed) [Member]
Cash equivalents:
Cash equivalents		21,219
Short-term investments:
Short-term investments	18,167	18,239
Fair Value, Measurements, Recurring [Member] | Quoted prices in active markets for identical assets (Level 1) [Member] | U.S. Treasury notes [Member]
Short-term investments:
Short-term investments	49,149	28,099
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member]
Cash equivalents:
Cash equivalents
Short-term investments:
Short-term investments
Total assets
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Certificates of deposit (U.S. Gov. guaranteed) [Member]
Short-term investments:
Short-term investments
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Money market funds [Member]
Cash equivalents:
Cash equivalents
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | U.S. Treasury bills [Member]
Cash equivalents:
Cash equivalents
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | Corporate bonds (U.S. Gov. guaranteed) [Member]
Cash equivalents:
Cash equivalents
Short-term investments:
Short-term investments
Fair Value, Measurements, Recurring [Member] | Significant other observable inputs (Level 2) [Member] | U.S. Treasury notes [Member]
Short-term investments:
Short-term investments
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member]
Cash equivalents:
Cash equivalents
Short-term investments:
Short-term investments
Total assets
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Certificates of deposit (U.S. Gov. guaranteed) [Member]
Short-term investments:
Short-term investments
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Money market funds [Member]
Cash equivalents:
Cash equivalents
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | U.S. Treasury bills [Member]
Cash equivalents:
Cash equivalents
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | Corporate bonds (U.S. Gov. guaranteed) [Member]
Cash equivalents:
Cash equivalents
Short-term investments:
Short-term investments
Fair Value, Measurements, Recurring [Member] | Significant unobservable inputs (Level 3) [Member] | U.S. Treasury notes [Member]
Short-term investments:
Short-term investments