Investment Securities - Securities Available-for-Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Sep. 30, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 719,607	$ 563,209
Gross unrealized gains	1,795	2,049
Gross unrealized (losses)	(10,832)	(8,082)
Securities available-for-sale, at fair value	710,570	557,176
U.S. government and agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	39,909	24,954
Gross unrealized gains	17	35
Gross unrealized (losses)	(282)	(116)
Securities available-for-sale, at fair value	39,644	24,873
Mortgage-backed securities and collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	615,924	479,927
Gross unrealized gains	383	652
Gross unrealized (losses)	(9,695)	(7,088)
Securities available-for-sale, at fair value	606,612	473,491
Municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,957	1,357
Gross unrealized gains	0	0
Gross unrealized (losses)	0
Securities available-for-sale, at fair value	1,957	1,357
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	54,489	49,489
Gross unrealized gains	536	536
Gross unrealized (losses)	(511)	(532)
Securities available-for-sale, at fair value	54,514	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)	(344)	(292)
Securities available-for-sale, at fair value	4,656	4,708
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,328	2,482
Gross unrealized gains	859	826
Gross unrealized (losses)	0	(54)
Securities available-for-sale, at fair value	$ 3,187	$ 3,254