Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017
Supplemental type certificates, accumulated amortization	$ 5,164	$ 4,345
Other assets, accumulated amortization	$ 8,213	$ 6,904
Preferred stock, par value (in dollars per share)	$ 5	$ 5
Preferred stock, shares authorized (in shares)	50,000,000	50,000,000
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)	66,196,854	65,273,896
Common stock, shares oustanding (in shares)	64,743,317	64,543,550
Treasury stock, shares (in shares)	1,453,537	730,346
Preferred Class A [Member]
Preferred stock, stated value (in dollars per share)	$ 100	$ 100
Preferred stock, dividend rate	9.80%	9.80%
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, designated classes (in shares)	200,000	200,000
Preferred Class B [Member]
Preferred stock, stated value (in dollars per share)	$ 1,000	$ 1,000
Preferred stock, dividend rate	6.00%	6.00%
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, designated classes (in shares)	200,000	200,000