Condensed Parent Company Financial Information - Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Thousands	Apr. 30, 2017	May 13, 2016	Apr. 30, 2016	Apr. 30, 2015	Apr. 30, 2014
Condensed Parent Company Balance Sheets
Total assets	$ 1,393,265		$ 1,240,814	$ 1,160,976
Stockholders' equity:
Common stock, par value $0.01 per share, authorized 500,000,000 shares; 40,970,905 and 32,892,905 shares issued at April 30, 2017 and April 30, 2016, respectively	410		329
Preferred stock, par value $0.01 per share, authorized 50,000,000 shares; 0 shares issued at April 30, 2017 and April 30, 2016, respectively
Additional paid-in capital	488,459		334,244
Retained earnings (accumulated deficit)	26,621		(22,265)
Accumulated other comprehensive loss	(884)		(1,148)
Total stockholders' equity	$ 514,606		$ 311,160	$ 299,572	$ 299,434
Stockholders' equity parenthetical
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01
Common stock, shares authorized	500,000,000	500,000,000	500,000,000
Common stock, shares issued	40,970,905		32,892,905
Common stock, shares outstanding	40,970,905		32,892,905
Preferred stock, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01
Preferred stock, shares authorized	50,000,000	50,000,000	50,000,000
Preferred stock, shares issued	0		0
Parent Company
Condensed Parent Company Balance Sheets
Investment in subsidiary	$ 514,606		$ 311,160
Total assets	514,606		311,160
Stockholders' equity:
Common stock, par value $0.01 per share, authorized 500,000,000 shares; 40,970,905 and 32,892,905 shares issued at April 30, 2017 and April 30, 2016, respectively	410		329
Preferred stock, par value $0.01 per share, authorized 50,000,000 shares; 0 shares issued at April 30, 2017 and April 30, 2016, respectively
Additional paid-in capital	488,459		334,244
Retained earnings (accumulated deficit)	26,621		(22,265)
Accumulated other comprehensive loss	(884)		(1,148)
Total stockholders' equity	$ 514,606		$ 311,160
Stockholders' equity parenthetical
Common stock, par value (in dollars per share)	$ 0.01		$ 0.01
Common stock, shares authorized	500,000,000		500,000,000
Common stock, shares issued	40,970,905		32,892,905
Common stock, shares outstanding	40,970,905		32,892,905
Preferred stock, par value (in dollars per share)	$ 0.01		$ 0.01
Preferred stock, shares authorized	50,000,000		50,000,000
Preferred stock, shares issued	0		0