Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	$ 3,683,521	$ 4,017,909
Available-for-sale Securities, Gross Unrealized Gain	135,415	31,812
Available-for-sale Securities, Gross Unrealized Loss	(1,588)	(40,290)
Securities available‑for‑sale	3,817,348	4,009,431
Government agency and government-sponsored enterprise pass through securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	313,935	281,486
Available-for-sale Securities, Gross Unrealized Gain	10,995	1,902
Available-for-sale Securities, Gross Unrealized Loss	(1)	(2,300)
Securities available‑for‑sale	324,929	281,088
Government agency and government-sponsored enterprise collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	1,141,332	634,774
Available-for-sale Securities, Gross Unrealized Gain	31,627	3,448
Available-for-sale Securities, Gross Unrealized Loss	(404)	(5,372)
Securities available‑for‑sale	1,172,555	632,850
Private label collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	89,859	101,313
Available-for-sale Securities, Gross Unrealized Gain	4,011	1,985
Available-for-sale Securities, Gross Unrealized Loss	(2)	(2,093)
Securities available‑for‑sale	93,868	101,205
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	675,645	1,298,514
Available-for-sale Securities, Gross Unrealized Gain	46,336	21,000
Available-for-sale Securities, Gross Unrealized Loss	0	(7,320)
Securities available‑for‑sale	721,981	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	1,063,659	1,133,846
Available-for-sale Securities, Gross Unrealized Gain	36,539	383
Available-for-sale Securities, Gross Unrealized Loss	(181)	(21,525)
Securities available‑for‑sale	1,100,017	1,112,704
US Treasury Securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	4,984	401,056
Available-for-sale Securities, Gross Unrealized Gain	219	2,437
Available-for-sale Securities, Gross Unrealized Loss	0	(88)
Securities available‑for‑sale	5,203	403,405
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	214,153	81,762
Available-for-sale Securities, Gross Unrealized Gain	700	104
Available-for-sale Securities, Gross Unrealized Loss	(702)	(481)
Securities available‑for‑sale	214,151	81,385
Collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	114,149
Available-for-sale Securities, Gross Unrealized Gain	23
Available-for-sale Securities, Gross Unrealized Loss	(298)
Securities available‑for‑sale	113,874
SBA asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	48,805	68,158
Available-for-sale Securities, Gross Unrealized Gain	941	0
Available-for-sale Securities, Gross Unrealized Loss	0	(1,111)
Securities available‑for‑sale	49,746	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	4,024	553
Available-for-sale Securities, Gross Unrealized Loss	0	0
Securities available‑for‑sale	$ 21,024	$ 17,553