Consolidated Balance Sheets (Parentheticals) - USD ($)	12 Months Ended
	Apr. 30, 2019	Apr. 30, 2018
Supplemental type certificates, accumulated amortization	$ 6,054,000	$ 5,164,000
Other assets, accumulated amortization	$ 9,370,000	$ 8,213,000
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)	71,008,122	66,196,854
Common stock, shares outstanding (in shares)	68,281,071	64,743,317
Treasury stock, shares (in shares)	2,727,051	1,453,537
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	$ 1,000,000	$ 1,000,000
Preferred stock, redemption value	$ 1,000,000	$ 1,000,000
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