Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	$ 3,705,268	$ 4,017,909
Available-for-sale Securities, Gross Unrealized Gain	103,919	31,812
Available-for-sale Securities, Gross Unrealized Loss	(1,943)	(40,290)
Securities available‑for‑sale	3,807,244	4,009,431
Government agency and government-sponsored enterprise pass through securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	343,131	281,486
Available-for-sale Securities, Gross Unrealized Gain	8,381	1,902
Available-for-sale Securities, Gross Unrealized Loss	(152)	(2,300)
Securities available‑for‑sale	351,360	281,088
Government agency and government-sponsored enterprise collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	1,125,267	634,774
Available-for-sale Securities, Gross Unrealized Gain	25,513	3,448
Available-for-sale Securities, Gross Unrealized Loss	(306)	(5,372)
Securities available‑for‑sale	1,150,474	632,850
Private label collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	108,526	101,313
Available-for-sale Securities, Gross Unrealized Gain	3,610	1,985
Available-for-sale Securities, Gross Unrealized Loss	(99)	(2,093)
Securities available‑for‑sale	112,037	101,205
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	700,470	1,298,514
Available-for-sale Securities, Gross Unrealized Gain	36,594	21,000
Available-for-sale Securities, Gross Unrealized Loss	(494)	(7,320)
Securities available‑for‑sale	736,570	1,312,194
Government agency and government-sponsored enterprise commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	1,033,895	1,133,846
Available-for-sale Securities, Gross Unrealized Gain	25,046	383
Available-for-sale Securities, Gross Unrealized Loss	(95)	(21,525)
Securities available‑for‑sale	1,058,846	1,112,704
US Treasury Securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	4,983	401,056
Available-for-sale Securities, Gross Unrealized Gain	205	2,437
Available-for-sale Securities, Gross Unrealized Loss	0	(88)
Securities available‑for‑sale	5,188	403,405
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	226,693	81,762
Available-for-sale Securities, Gross Unrealized Gain	866	104
Available-for-sale Securities, Gross Unrealized Loss	(438)	(481)
Securities available‑for‑sale	227,121	81,385
Collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	94,149	0
Available-for-sale Securities, Gross Unrealized Gain	0	0
Available-for-sale Securities, Gross Unrealized Loss	(347)	0
Securities available‑for‑sale	93,802	0
SBA asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	51,154	68,158
Available-for-sale Securities, Gross Unrealized Gain	670	0
Available-for-sale Securities, Gross Unrealized Loss	(12)	(1,111)
Securities available‑for‑sale	51,812	67,047
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	17,000	17,000
Available-for-sale Securities, Gross Unrealized Gain	3,034	553
Available-for-sale Securities, Gross Unrealized Loss	0	0
Securities available‑for‑sale	$ 20,034	$ 17,553