Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	$ 3,818,709	$ 3,721,525
Available-for-sale Securities, Gross Unrealized Gain	28,611	68,965
Available-for-sale Securities, Gross Unrealized Loss	(45,334)	(16,059)
Securities available‑for‑sale	3,801,986	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	251,585	243,375
Available-for-sale Securities, Gross Unrealized Gain	2,748	3,743
Available-for-sale Securities, Gross Unrealized Loss	(2,557)	(844)
Securities available‑for‑sale	251,776	246,274
Government agency and government-sponsored enterprise collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	551,021	277,638
Available-for-sale Securities, Gross Unrealized Gain	840	968
Available-for-sale Securities, Gross Unrealized Loss	(5,916)	(2,897)
Securities available‑for‑sale	545,945	275,709
Private label collateralized mortgage obligations
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	114,386	122,816
Available-for-sale Securities, Gross Unrealized Gain	3,071	3,813
Available-for-sale Securities, Gross Unrealized Loss	(1,212)	(642)
Securities available‑for‑sale	116,245	125,987
Municipal securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	1,392,261	1,627,707
Available-for-sale Securities, Gross Unrealized Gain	19,389	53,700
Available-for-sale Securities, Gross Unrealized Loss	(8,064)	(1,339)
Securities available‑for‑sale	1,403,586	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	1,114,430	1,169,969
Available-for-sale Securities, Gross Unrealized Gain	121	2,758
Available-for-sale Securities, Gross Unrealized Loss	(25,057)	(8,758)
Securities available‑for‑sale	1,089,494	1,163,969
US Treasury Securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	148,217	0
Available-for-sale Securities, Gross Unrealized Gain	365	0
Available-for-sale Securities, Gross Unrealized Loss	0	0
Securities available‑for‑sale	148,582	0
SBA asset-backed securities
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	148,787	160,214
Available-for-sale Securities, Gross Unrealized Gain	706	695
Available-for-sale Securities, Gross Unrealized Loss	(1,229)	(575)
Securities available‑for‑sale	148,264	160,334
Asset-backed
Schedule of Available-for-sale Securities [Line Items]
Total securities available-for-sale	81,022	89,425
Available-for-sale Securities, Gross Unrealized Gain	11	159
Available-for-sale Securities, Gross Unrealized Loss	(1,299)	(874)
Securities available‑for‑sale	79,734	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	1,360	2,295
Available-for-sale Securities, Gross Unrealized Loss	0	0
Securities available‑for‑sale	18,360	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