Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	$ 3,889,089	$ 3,721,525
Available-for-sale Securities, Gross Unrealized Gain	27,710	68,965
Available-for-sale Securities, Gross Unrealized Loss	(59,011)	(16,059)
Securities available‑for‑sale	3,857,788	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	263,915	243,375
Available-for-sale Securities, Gross Unrealized Gain	2,466	3,743
Available-for-sale Securities, Gross Unrealized Loss	(2,793)	(844)
Securities available‑for‑sale	263,588	246,274
Government agency and government-sponsored enterprise collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	562,970	277,638
Available-for-sale Securities, Gross Unrealized Gain	348	968
Available-for-sale Securities, Gross Unrealized Loss	(8,427)	(2,897)
Securities available‑for‑sale	554,891	275,709
Private label collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	101,082	122,816
Available-for-sale Securities, Gross Unrealized Gain	2,857	3,813
Available-for-sale Securities, Gross Unrealized Loss	(1,703)	(642)
Securities available‑for‑sale	102,236	125,987
Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	1,399,851	1,627,707
Available-for-sale Securities, Gross Unrealized Gain	20,374	53,700
Available-for-sale Securities, Gross Unrealized Loss	(8,133)	(1,339)
Securities available‑for‑sale	1,412,092	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	1,131,301	1,169,969
Available-for-sale Securities, Gross Unrealized Gain	0	2,758
Available-for-sale Securities, Gross Unrealized Loss	(34,085)	(8,758)
Securities available‑for‑sale	1,097,216	1,163,969
US Treasury Securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	262,558	0
Available-for-sale Securities, Gross Unrealized Gain	373	0
Available-for-sale Securities, Gross Unrealized Loss	(590)	0
Securities available‑for‑sale	262,341	0
SBA asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	79,317	160,214
Available-for-sale Securities, Gross Unrealized Gain	0	695
Available-for-sale Securities, Gross Unrealized Loss	(1,966)	(575)
Securities available‑for‑sale	77,351	160,334
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	71,095	89,425
Available-for-sale Securities, Gross Unrealized Gain	0	159
Available-for-sale Securities, Gross Unrealized Loss	(1,314)	(874)
Securities available‑for‑sale	69,781	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	1,292	2,295
Available-for-sale Securities, Gross Unrealized Loss	0	0
Securities available‑for‑sale	18,292	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