Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2017	Apr. 30, 2016
Supplemental type certificates, accumulated amortization	$ 4,345	$ 3,549
Other assets, accumulated amortization	$ 6,904	$ 5,579
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01
Common stock, shares authorized (in shares)	100,000,000	100,000,000
Common stock, shares issued (in shares)	65,273,896	64,066,873
Common stock, shares outstanding (in shares)	64,543,550	63,466,873
Treasury stock, shares (in shares)	730,346	600,000
Preferred Class A [Member]
Preferred stock, liquidation value	$ 100	$ 100
Preferred stock, redemption value	$ 100	$ 100
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Preferred stock, par value (in dollars per share)	$ 5	$ 5
Preferred stock, shares authorized (in shares)	50,000,000	50,000,000
Preferred stock, stated value (in dollars per share)	$ 100	$ 100
Preferred stock, dividend rate	9.80%	9.80%
Preferred Class B [Member]
Preferred stock, liquidation value	$ 1,000	$ 1,000
Preferred stock, redemption value	$ 1,000	$ 1,000
Preferred stock, shares issued (in shares)	0	0
Preferred stock, shares outstanding (in shares)	0	0
Preferred stock, par value (in dollars per share)	$ 5	$ 5
Preferred stock, shares authorized (in shares)	50,000,000	50,000,000
Preferred stock, stated value (in dollars per share)	$ 1,000	$ 1,000
Preferred stock, dividend rate	6.00%	6.00%
Preferred Class A and B [Member]
Preferred stock, designated classes (in shares)	200,000	200,000