Parent-only Financial Information - Statement of Financial Condition (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Sep. 30, 2016	Oct. 01, 2015	Sep. 30, 2015
Assets
Cash and due from banks	$ 42,065	$ 65,334
Short-term investments	136	164
Total cash and cash equivalents	42,201	65,498	$ 100,975	$ 45,694		$ 43,178
Securities available for sale, at fair value	1,032,868
Loans receivable, net	8,081	0
Other assets	107,048	89,668
Total assets	6,691,618	5,766,500
Liabilities:
Accrued expenses and other liabilities	84,475	76,495
Total liabilities	5,719,558	5,294,430
Total stockholders' equity	972,060	472,070	475,914	439,664		$ 417,998
Total liabilities and stockholders' equity	6,691,618	5,766,500
Parent Company
Assets
Cash and due from banks	153,697	1,537
Short-term investments	136	164
Total cash and cash equivalents	153,833	1,701	$ 1,598	$ 1,301	$ 2,631
Securities available for sale, at fair value	1,420	2,783
Investment in subsidiaries	764,663	516,607
Loans receivable, net	44,439	0
Other assets	7,852	3,281
Total assets	972,207	524,372
Liabilities:
Borrowings	0	50,657
Accrued expenses and other liabilities	147	1,645
Total liabilities	147	52,302
Total stockholders' equity	972,060	472,070
Total liabilities and stockholders' equity	$ 972,207	$ 524,372