Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	$ 3,881,727	$ 3,721,525
Available-for-sale Securities, Gross Unrealized Gain	17,072	68,965
Available-for-sale Securities, Gross Unrealized Loss	(78,466)	(16,059)
Securities available‑for‑sale	3,820,333	3,774,431
Government agency and government-sponsored enterprise pass through securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	251,150	243,375
Available-for-sale Securities, Gross Unrealized Gain	1,971	3,743
Available-for-sale Securities, Gross Unrealized Loss	(3,290)	(844)
Securities available‑for‑sale	249,831	246,274
Government agency and government-sponsored enterprise collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	583,062	277,638
Available-for-sale Securities, Gross Unrealized Gain	220	968
Available-for-sale Securities, Gross Unrealized Loss	(13,677)	(2,897)
Securities available‑for‑sale	569,605	275,709
Private label collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	110,353	122,816
Available-for-sale Securities, Gross Unrealized Gain	2,699	3,813
Available-for-sale Securities, Gross Unrealized Loss	(2,602)	(642)
Securities available‑for‑sale	110,450	125,987
Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	1,272,501	1,627,707
Available-for-sale Securities, Gross Unrealized Gain	10,974	53,700
Available-for-sale Securities, Gross Unrealized Loss	(11,892)	(1,339)
Securities available‑for‑sale	1,271,583	1,680,068
Government agency and government-sponsored enterprise commercial mortgage-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	1,117,749	1,169,969
Available-for-sale Securities, Gross Unrealized Gain	0	2,758
Available-for-sale Securities, Gross Unrealized Loss	(40,744)	(8,758)
Securities available‑for‑sale	1,077,005	1,163,969
US Treasury Securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	400,591	0
Available-for-sale Securities, Gross Unrealized Gain	0	0
Available-for-sale Securities, Gross Unrealized Loss	(2,960)	0
Securities available‑for‑sale	397,631	0
SBA asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	69,999	160,214
Available-for-sale Securities, Gross Unrealized Gain	0	695
Available-for-sale Securities, Gross Unrealized Loss	(2,413)	(575)
Securities available‑for‑sale	67,586	160,334
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	59,322	89,425
Available-for-sale Securities, Gross Unrealized Gain	18	159
Available-for-sale Securities, Gross Unrealized Loss	(888)	(874)
Securities available‑for‑sale	58,452	88,710
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	17,000	17,000
Available-for-sale Securities, Gross Unrealized Gain	1,190	2,295
Available-for-sale Securities, Gross Unrealized Loss	0	0
Securities available‑for‑sale	18,190	19,295
Collateralized loan obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	0	6,960
Available-for-sale Securities, Gross Unrealized Gain	0	55
Available-for-sale Securities, Gross Unrealized Loss	0	0
Securities available‑for‑sale	0	7,015
Equity Securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	0	6,421
Available-for-sale Securities, Gross Unrealized Gain	0	779
Available-for-sale Securities, Gross Unrealized Loss	0	(130)
Securities available‑for‑sale	$ 0	$ 7,070