Investments (Details) Share data in Millions, except Per Share data, unless otherwise specified			0 Months Ended	12 Months Ended						12 Months Ended
	Apr. 30, 2013 USD ($)	Apr. 30, 2012 USD ($)	May 14, 2013 CybAero	Apr. 30, 2013 CybAero Convertible bonds SEK	Apr. 30, 2013 Municipal securities USD ($)	Apr. 30, 2012 Municipal securities USD ($)	Apr. 30, 2012 U.S. government securities USD ($)	Apr. 30, 2013 Auction rate securities USD ($) item	Apr. 30, 2012 Auction rate securities USD ($) item	Apr. 30, 2013 Auction rate securities Minimum	Apr. 30, 2012 Auction rate securities Minimum	Apr. 30, 2013 Auction rate securities Maximum	Apr. 30, 2012 Auction rate securities Maximum	Apr. 30, 2013 Convertible bonds USD ($)	Apr. 30, 2013 Held-to-maturity securities Municipal securities USD ($)	Apr. 30, 2012 Held-to-maturity securities Municipal securities USD ($)	Apr. 30, 2012 Held-to-maturity securities U.S. government securities USD ($)	Apr. 30, 2013 Available-for-sale securities Auction rate securities USD ($)	Apr. 30, 2012 Available-for-sale securities Auction rate securities USD ($)	Apr. 30, 2013 Available-for-sale securities Convertible bonds USD ($) item
Short-term investments:
Total short-term investments	$ 73,241,000	$ 77,152,000													$ 73,241,000	$ 49,263,000	$ 27,889,000
Long-term investments:
Total long-term investments	68,916,000	58,457,000													54,158,000	52,261,000		5,687,000	6,196,000	9,071,000
Held-To-Maturity Securities
Amortized Cost	127,399,000	129,413,000			127,399,000	101,524,000	27,889,000
Gross Unrealized Gains	49,000	48,000			49,000	48,000
Gross Unrealized Losses	(23,000)	(25,000)			(23,000)	(24,000)	(1,000)
Fair Value	127,425,000	129,436,000			127,425,000	101,548,000	27,888,000
Amortized cost of held-to-maturity securities by contractual maturities
Due within one year	73,241,000
Due after one year through five years	54,158,000
Total	127,399,000
Fair value of held-to-maturity securities by contractual maturities
Due within one year	73,258,000
Due after one year through five years	54,167,000
Total	127,425,000
Available-For-Sale Securities
Number of available-for-sale securities								3	4
Maturity period of available-for-sale securities										6 years	6 years	21 years	21 years
Pre-determined interval to reset interest rates to current rates										30 days	30 days	35 days	35 days
Period for which the issuer of securities is not required to redeem the securities												365 days	365 days
Number of bonds held																				2
Available-For-Sale Securities
Amortized Cost								6,750,000	7,350,000					3,037,000
Gross Unrealized Gains														6,173,000
Gross Unrealized Losses								(1,063,000)	(1,154,000)					(139,000)
Total								5,687,000	6,196,000					9,071,000
Amortized cost of available-for-sale securities by contractual maturities
Due within five years														3,037,000
Due after five through 10 years								1,350,000
Due after 10 years								5,400,000
Total								6,750,000						3,037,000
Fair value of available-for-sale securities by contractual maturities
Due within five years														9,071,000
Due after five through 10 years								1,256,000
Due after 10 years								4,431,000
Total								5,687,000	6,196,000					9,071,000
Convertible bonds
Amount of each bond				10,000,000
Number of CybAero shares that will be received on conversion of notes				1
Conversion price (in SEK per share)				10
Annual interest rate (as a percent)				5.00%																5.00%
Notice period for prepayment of notes				3 months
Impact of reverse stock split on CybAero shares outstanding			0.1
Reverse stock split ratio effected by CybAero			one-for-10 reverse stock split