Fair Value Measurements (Details) (Recurring basis, USD $) In Thousands, unless otherwise specified	Apr. 30, 2014
Quoted prices in active market for identical assets (Level 1)
Fair Value Measurement
Available-for-sale securities	$ 5,309
Quoted prices in active market for identical assets (Level 1) | Equity securities
Fair Value Measurement
Available-for-sale securities	5,309
Significant other observable inputs (Level 2)
Fair Value Measurement
Available-for-sale securities	4,251
Significant other observable inputs (Level 2) | Convertible bonds
Fair Value Measurement
Available-for-sale securities	4,251
Significant unobservable inputs (Level 3)
Fair Value Measurement
Available-for-sale securities	7,297
Significant unobservable inputs (Level 3) | Auction rate securities
Fair Value Measurement
Available-for-sale securities	5,683
Significant unobservable inputs (Level 3) | Convertible bonds
Fair Value Measurement
Available-for-sale securities	1,614
Total Fair Value
Fair Value Measurement
Available-for-sale securities	16,857
Total Fair Value | Auction rate securities
Fair Value Measurement
Available-for-sale securities	5,683
Total Fair Value | Convertible bonds
Fair Value Measurement
Available-for-sale securities	5,865
Total Fair Value | Equity securities
Fair Value Measurement
Available-for-sale securities	$ 5,309