Investments (Details) SEK / shares in Units, SEK in Millions			3 Months Ended	6 Months Ended		12 Months Ended
	Aug. 31, 2015 USD ($)	Aug. 11, 2014 SEK SEK / shares shares	Aug. 01, 2015 USD ($)	Oct. 29, 2016 USD ($) item	Oct. 31, 2015 USD ($)	Apr. 30, 2016 USD ($) item	Aug. 31, 2015 SEK / shares	Apr. 30, 2015 item
Short-term investments:
Total short-term investments				$ 118,208,000		$ 103,404,000
Long-term investments:
Total long-term investments				42,559,000		33,859,000
Held To Maturity Securities
Amortized Cost				158,315,000		134,483,000
Gross Unrealized Gains				48,000		82,000
Gross Unrealized Losses				(100,000)		(39,000)
Fair Value				158,263,000		134,526,000
Amortized cost of held-to-maturity securities by contractual maturity
Due within one year				118,208,000
Due after one year through five years				40,107,000
Total				158,315,000		134,483,000
Fair value of held-to-maturity securities by contractual maturity
Due within one year				118,224,000
Due after one year through five years				40,039,000
Fair Value				158,263,000		134,526,000
Available-For-Sale Securities
Amortized Cost				2,700,000		3,100,000
Gross Unrealized Losses				(248,000)		(320,000)
Total				2,452,000		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				2,452,000		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
Amortized Cost				59,531,000		42,179,000
Gross Unrealized Gains				7,000		5,000
Gross Unrealized Losses				(28,000)		(7,000)
Fair Value				59,510,000		42,177,000
Amortized cost of held-to-maturity securities by contractual maturity
Total				59,531,000		42,179,000
Fair value of held-to-maturity securities by contractual maturity
Fair Value				59,510,000		42,177,000
U.S. government securities
Held To Maturity Securities
Amortized Cost				27,682,000		28,702,000
Gross Unrealized Gains				18,000		23,000
Gross Unrealized Losses				(1,000)
Fair Value				27,699,000		28,725,000
Amortized cost of held-to-maturity securities by contractual maturity
Total				27,682,000		28,702,000
Fair value of held-to-maturity securities by contractual maturity
Fair Value				27,699,000		28,725,000
Corporate bonds
Held To Maturity Securities
Amortized Cost				66,602,000		63,602,000
Gross Unrealized Gains				23,000		54,000
Gross Unrealized Losses				(70,000)		(32,000)
Fair Value				66,555,000		63,624,000
Amortized cost of held-to-maturity securities by contractual maturity
Total				66,602,000		63,602,000
Fair value of held-to-maturity securities by contractual maturity
Fair Value				66,555,000		63,624,000
Certificates of deposit
Held To Maturity Securities
Amortized Cost				4,500,000
Gross Unrealized Losses				(1,000)
Fair Value				4,499,000
Amortized cost of held-to-maturity securities by contractual maturity
Total				4,500,000
Fair value of held-to-maturity securities by contractual maturity
Fair Value				4,499,000
Auction rate securities
Available-For-Sale Securities
Amortized Cost				2,700,000		3,100,000
Gross Unrealized Losses				(248,000)		(320,000)
Total				2,452,000		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				679,000
Due after 10 years				1,773,000
Total				$ 2,452,000		$ 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
Convertible bonds | CybAero | Exercise of conversion right on convertible debt securities
Equity Securities
Face amount converted | SEK		SEK 10
Conversion price (in SEK per share) | SEK / shares		SEK 9.41
Fair value of shares (in SEK per share) | SEK / shares		SEK 37.50
Held-to-maturity securities
Short-term investments:
Total short-term investments				$ 118,208,000		$ 103,404,000
Long-term investments:
Total long-term investments				40,107,000		31,079,000
Held-to-maturity securities | Municipal securities
Short-term investments:
Total short-term investments				49,695,000		42,179,000
Long-term investments:
Total long-term investments				9,836,000
Held-to-maturity securities | U.S. government securities
Short-term investments:
Total short-term investments				23,664,000		21,184,000
Long-term investments:
Total long-term investments				4,018,000		7,518,000
Held-to-maturity securities | Corporate bonds
Short-term investments:
Total short-term investments				40,349,000		40,041,000
Long-term investments:
Total long-term investments				26,253,000		23,561,000
Held-to-maturity securities | Certificates of deposit
Short-term investments:
Total short-term investments				4,500,000
Available-for-sale securities
Long-term investments:
Total long-term investments				2,452,000		2,780,000
Available-for-sale securities | Auction rate securities
Long-term investments:
Total long-term investments				$ 2,452,000		$ 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