Investments (Details) Share data in Millions, except Per Share data, unless otherwise specified			0 Months Ended	6 Months Ended					6 Months Ended															6 Months Ended
	Oct. 26, 2013 USD ($)	Apr. 30, 2013 USD ($)	May 14, 2013 CybAero	Oct. 26, 2013 CybAero Convertible bonds SEK	Oct. 26, 2013 Municipal securities USD ($)	Apr. 30, 2013 Municipal securities USD ($)	Oct. 26, 2013 Auction rate securities USD ($) item	Apr. 30, 2013 Auction rate securities USD ($)	Oct. 26, 2013 Auction rate securities Minimum	Oct. 26, 2013 Auction rate securities Maximum	Oct. 26, 2013 Convertible bonds USD ($)	Apr. 30, 2013 Convertible bonds USD ($)	Oct. 26, 2013 Certificates of deposit USD ($)	Oct. 26, 2013 Held-to-maturity securities USD ($)	Apr. 30, 2013 Held-to-maturity securities USD ($)	Oct. 26, 2013 Held-to-maturity securities Municipal securities USD ($)	Apr. 30, 2013 Held-to-maturity securities Municipal securities USD ($)	Oct. 26, 2013 Held-to-maturity securities Certificates of deposit USD ($)	Oct. 26, 2013 Available-for-sale securities USD ($)	Apr. 30, 2013 Available-for-sale securities USD ($)	Oct. 26, 2013 Available-for-sale securities CybAero Convertible bonds item	Oct. 26, 2013 Available-for-sale securities Auction rate securities USD ($)	Apr. 30, 2013 Available-for-sale securities Auction rate securities USD ($)	Oct. 26, 2013 Available-for-sale securities Auction rate securities Maximum	Oct. 26, 2013 Available-for-sale securities Convertible bonds USD ($)	Apr. 30, 2013 Available-for-sale securities Convertible bonds USD ($)
Short-term investments:
Total short-term investments	$ 77,677,000	$ 73,241,000														$ 77,677,000	$ 73,241,000
Long-term investments:
Total long-term investments	51,707,000	68,916,000												42,789,000	54,158,000	38,898,000	54,158,000	3,891,000	8,918,000	14,758,000		5,467,000	5,687,000		3,451,000	9,071,000
Held-To-Maturity Securities
Amortized Cost	120,466,000	127,399,000			116,575,000	127,399,000							3,891,000
Gross Unrealized Gains	75,000	49,000			75,000	49,000
Gross Unrealized Losses	(11,000)	(23,000)			(11,000)	(23,000)
Fair Value	120,530,000	127,425,000			116,639,000	127,425,000							3,891,000
Amortized cost of held-to-maturity securities by contractual maturity
Due within one year	77,677,000
Due after one year through three years	42,789,000
Total	120,466,000
Fair value of held-to-maturity securities by contractual maturity
Due within one year	77,711,000
Due after one year through three years	42,819,000
Total	120,530,000
Available-For-Sale Securities
Number of available-for-sale securities							3
Maturity period of available-for-sale securities									6 years	21 years
Pre-determined interval to reset interest rates to current rates									30 days	35 days
Period for which the issuer of securities is not required to redeem the securities																								365 days
Available-For-Sale Securities
Amortized Cost							6,575,000	6,750,000			3,037,000	3,037,000
Gross Unrealized Gains											461,000	6,173,000
Gross Unrealized Losses							(1,108,000)	(1,063,000)			(47,000)	(139,000)
Total							5,467,000	5,687,000			3,451,000	9,071,000
Amortized cost of available-for-sale securities by contractual maturity
Due within five years											3,037,000
Due after five through 10 years							1,300,000
Due after 10 years							5,275,000
Total							6,575,000				3,037,000
Fair value of available-for-sale securities by contractual maturity
Due within five years											3,451,000
Due after five through 10 years							1,203,000
Due after 10 years							4,264,000
Total							5,467,000	5,687,000			3,451,000	9,071,000
Convertible bonds
Number of bonds held																					2
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%
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