Investments (Details) Share data in Millions, except Per Share data, unless otherwise specified			0 Months Ended	3 Months Ended					3 Months Ended									3 Months Ended
	Jul. 27, 2013 USD ($)	Apr. 30, 2013 USD ($)	May 14, 2013 CybAero	Jul. 27, 2013 CybAero Convertible bonds SEK	Jul. 27, 2013 Municipal securities USD ($)	Apr. 30, 2013 Municipal securities USD ($)	Jul. 27, 2013 Auction rate securities USD ($) item	Apr. 30, 2013 Auction rate securities USD ($)	Jul. 27, 2013 Auction rate securities Minimum	Jul. 27, 2013 Auction rate securities Maximum	Jul. 27, 2013 Convertible bonds USD ($)	Apr. 30, 2013 Convertible bonds USD ($)	Jul. 27, 2013 Held-to-maturity securities Municipal securities USD ($)	Apr. 30, 2013 Held-to-maturity securities Municipal securities USD ($)	Jul. 27, 2013 Available-for-sale securities CybAero Convertible bonds item	Jul. 27, 2013 Available-for-sale securities Auction rate securities USD ($)	Apr. 30, 2013 Available-for-sale securities Auction rate securities USD ($)	Jul. 27, 2013 Available-for-sale securities Auction rate securities Maximum	Jul. 27, 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	$ 64,474,000	$ 73,241,000											$ 64,474,000	$ 73,241,000
Long-term investments:
Total long-term investments	67,595,000	68,916,000											56,484,000	54,158,000		5,444,000	5,687,000		5,667,000	9,071,000
Held-To-Maturity Securities
Amortized Cost	120,958,000	127,399,000			120,958,000	127,399,000
Gross Unrealized Gains	75,000	49,000			75,000	49,000
Gross Unrealized Losses	(32,000)	(23,000)			(32,000)	(23,000)
Fair Value	121,001,000	127,425,000			121,001,000	127,425,000
Amortized cost of held-to-maturity securities by contractual maturity
Due within one year	64,474,000
Due after one year through three years	56,484,000
Total	120,958,000
Fair value of held-to-maturity securities by contractual maturity
Due within one year	64,503,000
Due after one year through three years	56,498,000
Total	121,001,000
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											2,781,000	6,173,000
Gross Unrealized Losses							(1,131,000)	(1,063,000)			(151,000)	(139,000)
Total							5,444,000	5,687,000			5,667,000	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							1,350,000
Due after 10 years							5,225,000
Total							6,575,000				3,037,000
Fair value of available-for-sale securities by contractual maturity
Due within five years											5,667,000
Due after five through 10 years							1,242,000
Due after 10 years							4,202,000
Total							5,444,000	5,687,000			5,667,000	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