Condensed Parent Company Financial Information - Statements of Operations and Comprehensive Income (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Apr. 30, 2018	Jan. 31, 2018	Oct. 31, 2017	Jul. 31, 2017	Apr. 30, 2017	Jan. 31, 2017	Oct. 31, 2016	Jul. 31, 2016	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Condensed Parent Company Statements of Operations and Comprehensive Income
Net income	$ 205,792	$ 195,420	$ 212,260	$ 205,104	$ 14,273	$ 8,226	$ 17,224	$ 9,163	$ 62,971	$ 48,886	$ 12,564
Comprehensive income									$ 64,296	$ 49,150	$ 11,406
Weighted average common shares outstanding:
Basic (in shares)					40,956	40,943	40,943	38,201	41,015,000	40,260,000	32,799,000
Diluted (in shares)	41,048	41,036	41,006	40,971	41,759	41,578	41,320	38,602	42,163,000	41,070,000	33,125,000
Net income per share:
Basic (in dollars per share)					$ 0.35	$ 0.20	$ 0.42	$ 0.24	$ 1.54	$ 1.21	$ 0.38
Diluted (in dollars per share)	$ 0.24	$ 0.48	$ 0.44	$ 0.37	$ 0.33	$ 0.20	$ 0.42	$ 0.24	$ 1.49	$ 1.19	$ 0.38
Parent Company
Condensed Parent Company Statements of Operations and Comprehensive Income
Net income in subsidiaries									$ 62,971	$ 48,886	$ 12,564
Net income									62,971	48,886	12,564
Comprehensive income									$ 64,296	$ 49,150	$ 11,406
Weighted average common shares outstanding:
Basic (in shares)									41,015,000	40,260,000	32,799,000
Diluted (in shares)									42,163,000	41,070,000	33,125,000
Net income per share:
Basic (in dollars per share)									$ 1.54	$ 1.21	$ 0.38
Diluted (in dollars per share)									$ 1.49	$ 1.19	$ 0.38