Investment Securities - Securities Available-for-Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 872,774	$ 563,209
Gross unrealized gains	1,532	2,049
Gross unrealized (losses)	(22,093)	(8,082)
Securities available-for-sale, at fair value	852,213	557,176
U.S. government and agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	54,799	24,954
Gross unrealized gains	26	35
Gross unrealized (losses)	(928)	(116)
Securities available-for-sale, at fair value	53,897	24,873
Mortgage-backed securities and collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	754,862	479,927
Gross unrealized gains	406	652
Gross unrealized (losses)	(20,069)	(7,088)
Securities available-for-sale, at fair value	735,199	473,491
Municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,295	1,357
Gross unrealized gains	0	0
Gross unrealized (losses)	0	0
Securities available-for-sale, at fair value	1,295	1,357
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	54,490	49,489
Gross unrealized gains	270	536
Gross unrealized (losses)	(748)	(532)
Securities available-for-sale, at fair value	54,012	49,493
Trust preferred securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	5,000	5,000
Gross unrealized gains	0	0
Gross unrealized (losses)	(348)	(292)
Securities available-for-sale, at fair value	4,652	4,708
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,328	2,482
Gross unrealized gains	830	826
Gross unrealized (losses)	0	(54)
Securities available-for-sale, at fair value	$ 3,158	$ 3,254