Condensed Parent Company Financial Information - Balance Sheets (Details) - USD ($) $ / shares in Units, $ in Thousands	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016	Apr. 30, 2015
Condensed Parent Company Balance Sheets
Total assets	$ 1,454,511	$ 1,393,265	$ 1,240,814
Stockholders’ equity:
Common stock, $0.01 par value, authorized 500,000 shares; 41,069 and 40,971 shares issued and outstanding as of April 30, 2018 and 2017, respectively	411	410
Additional paid-in capital	489,007	488,459
Accumulated deficit	89,592	26,621
Accumulated other comprehensive income	441	(884)
Total stockholders’ equity	$ 579,451	$ 514,606	$ 311,160	$ 299,572
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	41,069,000	40,971,000
Common stock, shares outstanding	41,069,000	40,971,000
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
Parent Company
Condensed Parent Company Balance Sheets
Investment in subsidiary	$ 579,451	$ 514,606
Total assets	579,451	514,606
Stockholders’ equity:
Common stock, $0.01 par value, authorized 500,000 shares; 41,069 and 40,971 shares issued and outstanding as of April 30, 2018 and 2017, respectively	411	410
Additional paid-in capital	489,007	488,459
Accumulated deficit	89,592	26,621
Accumulated other comprehensive income	441	(884)
Total stockholders’ equity	$ 579,451	$ 514,606
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	41,069,000	40,971,000
Common stock, shares outstanding	41,069,000	40,971,000