Investments (Details) SEK / shares in Units, SEK in Millions		1 Months Ended	3 Months Ended		12 Months Ended
	Aug. 11, 2014 SEK SEK / shares shares	Aug. 31, 2015 USD ($)	Jul. 30, 2016 USD ($) item	Aug. 01, 2015 USD ($)	Apr. 30, 2016 USD ($) item	Aug. 31, 2015 SEK / shares	Apr. 30, 2015 item
Short-term investments:
Total short-term investments			$ 101,802,000		$ 103,404,000
Long-term investments:
Total long-term investments			33,097,000		33,859,000
Held To Maturity Securities
Cost			132,489,000		134,483,000
Gross Unrealized Gains					82,000
Gross Unrealized Losses					(39,000)
Fair Value			132,594,000		134,526,000
Amortized cost of held-to-maturity securities by contractual maturity
Due within one year			101,802,000
Due after one year through five years			30,687,000
Total			132,489,000		134,483,000
Fair value of held-to-maturity securities by contractual maturity
Due within one year			101,855,000
Due after one year through five years			30,739,000
Fair Value			132,594,000		134,526,000
Available-For-Sale Securities
Amortized Cost			2,700,000		3,100,000
Gross Unrealized Losses			(290,000)		(320,000)
Total			2,410,000		2,780,000
Other than temporary impairment of available-for-sale securities				$ 2,186,000
Amortized cost of available-for-sale securities by contractual maturity
Total			2,700,000		3,100,000
Fair value of available-for-sale securities by contractual maturity
Total			2,410,000		2,780,000
Equity Securities
Shares received on conversion | shares	1,062,699
CybAero | Private Placement
Equity Securities
Share price (in SEK per share) | SEK / shares						SEK 12.00
Proceeds received from sale of stock		$ 777,000
Realized gains on sale of shares		$ 207,000
CybAero | Other Expense
Available-For-Sale Securities
Other than temporary impairment of available-for-sale securities				$ 2,186,000
Municipal securities
Held To Maturity Securities
Cost			38,859,000		42,179,000
Gross Unrealized Gains			24,000		5,000
Gross Unrealized Losses					(7,000)
Fair Value			38,883,000		42,177,000
Amortized cost of held-to-maturity securities by contractual maturity
Total			38,859,000		42,179,000
Fair value of held-to-maturity securities by contractual maturity
Fair Value			38,883,000		42,177,000
U.S. government securities
Held To Maturity Securities
Cost			27,182,000		28,702,000
Gross Unrealized Gains			27,000		23,000
Fair Value			27,209,000		28,725,000
Amortized cost of held-to-maturity securities by contractual maturity
Total			27,182,000		28,702,000
Fair value of held-to-maturity securities by contractual maturity
Fair Value			27,209,000		28,725,000
Corporate bonds
Held To Maturity Securities
Cost			66,448,000		63,602,000
Gross Unrealized Gains			66,000		54,000
Gross Unrealized Losses			(12,000)		(32,000)
Fair Value			66,502,000		63,624,000
Amortized cost of held-to-maturity securities by contractual maturity
Total			66,448,000		63,602,000
Fair value of held-to-maturity securities by contractual maturity
Fair Value			66,502,000		63,624,000
Auction rate securities
Available-For-Sale Securities
Amortized Cost			2,700,000		3,100,000
Gross Unrealized Losses			(290,000)		(320,000)
Total			2,410,000		2,780,000
Amortized cost of available-for-sale securities by contractual maturity
Due one through five years			700,000
Due after 10 years			2,000,000
Total			2,700,000		3,100,000
Fair value of available-for-sale securities by contractual maturity
Due one through five years			675,000
Due after 10 years			1,735,000
Total			$ 2,410,000		$ 2,780,000
Equity Securities
Number of available-for-sale securities | item			2		2
Auction rate securities | Minimum
Available For Sale Securities
Maturity period of available-for-sale securities			3 years		3 years
Pre-determined interval to reset interest rates to current rates			30 days		30 days
Auction rate securities | Maximum
Available For Sale Securities
Maturity period of available-for-sale securities			18 years		18 years
Pre-determined interval to reset interest rates to current rates			35 days		35 days
Certificates of deposit
Held To Maturity Securities
Cost			$ 132,489,000
Gross Unrealized Gains			117,000
Gross Unrealized Losses			(12,000)
Fair Value			132,594,000
Amortized cost of held-to-maturity securities by contractual maturity
Total			132,489,000
Fair value of held-to-maturity securities by contractual maturity
Fair Value			132,594,000
Convertible bonds | CybAero | Exercise of conversion right on convertible debt securities
Equity Securities
Conversion price (in SEK per share) | SEK / shares	SEK 9.41
Fair value of shares (in SEK per share) | SEK / shares	SEK 37.50
Face amount converted | SEK	SEK 10
Held-to-maturity securities
Short-term investments:
Total short-term investments			101,802,000		$ 103,404,000
Long-term investments:
Total long-term investments			30,687,000		31,079,000
Held-to-maturity securities | Municipal securities
Short-term investments:
Total short-term investments			35,121,000		42,179,000
Long-term investments:
Total long-term investments			3,738,000
Held-to-maturity securities | U.S. government securities
Short-term investments:
Total short-term investments			23,176,000		21,184,000
Long-term investments:
Total long-term investments			4,006,000		7,518,000
Held-to-maturity securities | Corporate bonds
Short-term investments:
Total short-term investments			43,505,000		40,041,000
Long-term investments:
Total long-term investments			22,943,000		23,561,000
Available-for-sale securities
Long-term investments:
Total long-term investments			2,410,000		2,780,000
Available-for-sale securities | Auction rate securities
Long-term investments:
Total long-term investments			$ 2,410,000		$ 2,780,000
Available-for-sale securities | Auction rate securities | Maximum
Available For Sale Securities
Period for which the issuer of securities is not required to redeem the securities			365 days		365 days
Available-for-sale securities | Common Stock | CybAero
Equity Securities
Number of available-for-sale securities | item							618,042