Investments (Details)			1 Months Ended	12 Months Ended
	Aug. 11, 2014 SEK / shares shares	Aug. 01, 2014 USD ($)	Aug. 31, 2014 USD ($)	Apr. 30, 2016 USD ($) item	Apr. 30, 2015 USD ($) shares	Aug. 31, 2014 SEK / shares
Short-term investments:
Total short-term investments				$ 103,404,000	$ 85,381,000
Long-term investments:
Total long-term investments				33,859,000	46,769,000
Held To Maturity Securities
Cost				134,483,000	127,836,000
Gross Unrealized Gains				82,000	20,000
Gross Unrealized Losses				(39,000)	(48,000)
Fair Value				134,526,000	127,808,000
Amortized cost of held-to-maturity securities by contractual maturity
Due within one year				103,404,000
Due after one year through five years				31,079,000
Total				134,483,000	127,836,000
Fair value of held-to-maturity securities by contractual maturity
Due within one year				103,415,000
Due after one year through five years				31,111,000
Fair Value				134,526,000	127,808,000
Available-For-Sale Securities
Other than temporary impairment of available-for-sale securities				2,186,000
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	4,784,000
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				42,179,000	97,591,000
Gross Unrealized Gains				5,000	8,000
Gross Unrealized Losses				(7,000)	(35,000)
Fair Value				42,177,000	97,564,000
Amortized cost of held-to-maturity securities by contractual maturity
Total				42,179,000	97,591,000
Fair value of held-to-maturity securities by contractual maturity
Fair Value				42,177,000	97,564,000
U.S. government securities
Held To Maturity Securities
Cost				28,702,000	16,545,000
Gross Unrealized Gains				23,000	12,000
Fair Value				28,725,000	16,557,000
Amortized cost of held-to-maturity securities by contractual maturity
Total				28,702,000	16,545,000
Fair value of held-to-maturity securities by contractual maturity
Fair Value				28,725,000	16,557,000
Corporate bonds
Held To Maturity Securities
Cost				63,602,000	9,815,000
Gross Unrealized Gains				54,000
Gross Unrealized Losses				(32,000)	(13,000)
Fair Value				63,624,000	9,802,000
Amortized cost of held-to-maturity securities by contractual maturity
Total				63,602,000	9,815,000
Fair value of held-to-maturity securities by contractual maturity
Fair Value				63,624,000	9,802,000
Auction rate securities
Available-For-Sale Securities
Amortized Cost				3,100,000	3,200,000
Gross Unrealized Losses				(320,000)	(359,000)
Total				2,780,000	2,841,000
Amortized cost of available-for-sale securities by contractual maturity
Due within five years				1,100,000
Due after 10 years				2,000,000
Total				3,100,000	3,200,000
Fair value of available-for-sale securities by contractual maturity
Due within five years				1,060,000
Due after 10 years				1,720,000
Total				$ 2,780,000	$ 2,841,000
Equity Securities
Number of available-for-sale securities | item				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					$ 3,885,000
Fair Value					3,885,000
Amortized cost of held-to-maturity securities by contractual maturity
Total					3,885,000
Fair value of held-to-maturity securities by contractual maturity
Fair Value					3,885,000
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
Equity securities
Available-For-Sale Securities
Amortized Cost					3,357,000
Gross Unrealized Losses					(1,884,000)
Total					1,473,000
Amortized cost of available-for-sale securities by contractual maturity
Total					3,357,000
Fair value of available-for-sale securities by contractual maturity
Total					1,473,000
Held-to-maturity securities
Short-term investments:
Total short-term investments				$ 103,404,000	83,908,000
Long-term investments:
Total long-term investments				31,079,000	43,928,000
Held-to-maturity securities | Municipal securities
Short-term investments:
Total short-term investments				42,179,000	67,173,000
Long-term investments:
Total long-term investments					30,418,000
Held-to-maturity securities | U.S. government securities
Short-term investments:
Total short-term investments				21,184,000	11,536,000
Long-term investments:
Total long-term investments				7,518,000	5,009,000
Held-to-maturity securities | Corporate bonds
Short-term investments:
Total short-term investments				40,041,000	1,314,000
Long-term investments:
Total long-term investments				23,561,000	8,501,000
Held-to-maturity securities | Certificates of deposit
Short-term investments:
Total short-term investments					3,885,000
Available-for-sale securities
Short-term investments:
Total short-term investments					1,473,000
Long-term investments:
Total long-term investments				2,780,000	2,841,000
Available-for-sale securities | Auction rate securities
Long-term investments:
Total long-term investments				$ 2,780,000	$ 2,841,000
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 | Equity securities
Short-term investments:
Total short-term investments					$ 1,473,000
Available-for-sale securities | Common Stock | CybAero
Equity Securities
Securities availalble for sale | shares					618,042