Investment Securities - Securities Available-for-Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 1,019,247	$ 719,607
Gross unrealized gains	1,206	1,795
Gross unrealized (losses)	(33,377)	(10,832)
Securities available-for-sale, at fair value	987,076	710,570
U.S. government and agency obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	54,813	39,909
Gross unrealized gains	0	17
Gross unrealized (losses)	(1,650)	(282)
Securities available-for-sale, at fair value	53,163	39,644
Mortgage-backed securities and collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	902,572	615,924
Gross unrealized gains	321	383
Gross unrealized (losses)	(30,207)	(9,695)
Securities available-for-sale, at fair value	872,686	606,612
Municipal obligations
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,587	1,957
Gross unrealized gains	0	0
Gross unrealized (losses)	0	0
Securities available-for-sale, at fair value	1,587	1,957
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	54,492	54,489
Gross unrealized gains	13	536
Gross unrealized (losses)	(1,113)	(511)
Securities available-for-sale, at fair value	53,392	54,514
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)	(407)	(344)
Securities available-for-sale, at fair value	4,593	4,656
Equity securities
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	783	2,328
Gross unrealized gains	872	859
Gross unrealized (losses)	0	0
Securities available-for-sale, at fair value	$ 1,655	$ 3,187