Investments (Details)			0 Months Ended		9 Months Ended		12 Months Ended							12 Months Ended					0 Months Ended	12 Months Ended										12 Months Ended						0 Months Ended
	Apr. 30, 2014 USD ($)	Apr. 30, 2013 USD ($)	May 14, 2013 CybAero	Feb. 12, 2014 CybAero Convertible bonds SEK	Jan. 25, 2014 CybAero Convertible bonds	Jan. 25, 2014 CybAero Convertible bonds SEK	Apr. 30, 2014 CybAero Convertible bonds	Apr. 30, 2014 CybAero Convertible bonds SEK	Apr. 30, 2013 CybAero Convertible bonds USD ($)	Apr. 30, 2014 Municipal securities USD ($)	Apr. 30, 2013 Municipal securities USD ($)	Apr. 30, 2014 Auction rate securities USD ($) bond	Apr. 30, 2013 Auction rate securities USD ($) bond	Apr. 30, 2014 Auction rate securities Minimum	Apr. 30, 2014 Auction rate securities Maximum	Apr. 30, 2014 Certificates of deposit USD ($)	Apr. 30, 2014 Convertible bonds USD ($)	Apr. 30, 2013 Convertible bonds USD ($)	Feb. 28, 2014 Convertible bonds CybAero Exercise of conversion right on convertible debt securities SEK bond	Apr. 30, 2014 Common stock CybAero USD ($)	Apr. 30, 2014 Held-to-maturity securities USD ($)	Apr. 30, 2013 Held-to-maturity securities USD ($)	Apr. 30, 2014 Held-to-maturity securities Municipal securities USD ($)	Apr. 30, 2013 Held-to-maturity securities Municipal securities USD ($)	Apr. 30, 2014 Held-to-maturity securities Certificates of deposit USD ($)	Apr. 30, 2014 Available-for-sale securities USD ($)	Apr. 30, 2013 Available-for-sale securities USD ($)	Apr. 30, 2014 Available-for-sale securities Auction rate securities USD ($)	Apr. 30, 2013 Available-for-sale securities Auction rate securities USD ($)	Apr. 30, 2014 Available-for-sale securities Auction rate securities Maximum	Apr. 30, 2014 Available-for-sale securities Convertible bonds USD ($)	Apr. 30, 2013 Available-for-sale securities Convertible bonds USD ($)	Apr. 30, 2014 Available-for-sale securities Convertible bonds CybAero bond	Apr. 30, 2013 Available-for-sale securities Convertible bonds CybAero bond	Apr. 30, 2014 Available-for-sale securities Equity securities USD ($)	Feb. 28, 2014 Available-for-sale securities Common stock CybAero Exercise of conversion right on convertible debt securities
Short-term investments:
Total short-term investments	$ 70,639,000	$ 73,241,000																					$ 69,898,000	$ 73,241,000	$ 741,000
Long-term investments:
Total long-term investments	50,505,000	68,916,000																			33,648,000	54,158,000	29,759,000	54,158,000	3,889,000	16,857,000	14,758,000	5,683,000	5,687,000		5,865,000	9,071,000			5,309,000
Held-To-Maturity Securities
Amortized Cost	104,287,000	127,399,000								99,657,000	127,399,000					4,630,000
Gross Unrealized Gains	65,000	49,000								65,000	49,000
Gross Unrealized Losses	(9,000)	(23,000)								(9,000)	(23,000)
Fair Value	104,343,000	127,425,000								99,713,000	127,425,000					4,630,000
Amortized cost of held-to-maturity securities by contractual maturity
Due within one year	70,639,000
Due after one year through five years	33,648,000
Total	104,287,000	127,399,000								99,657,000	127,399,000					4,630,000
Fair value of held-to-maturity securities by contractual maturities
Due within one year	70,671,000
Due after one year through five years	33,672,000
Fair Value	104,343,000	127,425,000								99,713,000	127,425,000					4,630,000
Available-For-Sale Securities
Number of available-for-sale securities												3	4
Maturity period of available-for-sale securities														5 years	20 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
Number of bonds held																																	1	2
Face amount converted																			10,000,000
Shares received on conversion																																				1,062,699
Available-For-Sale Securities
Amortized Cost												6,575,000	6,750,000				1,519,000	3,037,000
Gross Unrealized Gains																	4,346,000	6,173,000
Gross Unrealized Losses												(892,000)	(1,063,000)					(139,000)
Total												5,683,000	5,687,000				5,865,000	9,071,000
Amortized cost of available-for-sale securities by contractual maturities
Due within five years																	1,519,000
Due after five through 10 years												1,300,000
Due after 10 years												5,275,000
Total												6,575,000					1,519,000
Fair value of available-for-sale securities by contractual maturities
Due within five years																	5,865,000
Due after five through 10 years												1,209,000
Due after 10 years												4,474,000
Total												5,683,000	5,687,000				5,865,000	9,071,000
Convertible bonds
Amount of each bond								10,000,000
Number of CybAero shares that will be received on conversion of notes				1,062,699	1,000,000	1,000,000			1,000,000
Conversion price (in SEK per share)				9.41		10			$ 10										9.41
Annual interest rate (as a percent)							5.00%	5.00%
Notice period for prepayment of notes							3 months	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
Number of bonds converted																			1
Realized gains on sale of shares																				$ 132,000