Investment Securities - Securities Available for Sale (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	$ 6,997,535	$ 4,996,970
Gross Unrealized Gains	214,682	241,999
Gross Unrealized Losses	(13,609)	(3,378)
Securities available-for-sale	7,198,608	5,235,591
Government agency and government-sponsored enterprise pass through securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	1,203,537	1,207,676
Gross Unrealized Gains	56,261	74,238
Gross Unrealized Losses	(2,170)	(37)
Securities available-for-sale	1,257,628	1,281,877
Government agency and government-sponsored enterprise collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	1,108,587	1,172,166
Gross Unrealized Gains	36,736	47,994
Gross Unrealized Losses	(2,925)	(280)
Securities available-for-sale	1,142,398	1,219,880
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	1,735,937	1,438,004
Gross Unrealized Gains	83,607	93,631
Gross Unrealized Losses	(2,045)	(18)
Securities available-for-sale	1,817,499	1,531,617
Agency residential MBS
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	544,762	329,488
Gross Unrealized Gains	11,208	12,483
Gross Unrealized Losses	(3,559)	(897)
Securities available-for-sale	552,411	341,074
Private label commercial MBS
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	375,184	81,878
Gross Unrealized Gains	3,126	1,089
Gross Unrealized Losses	(75)	(10)
Securities available-for-sale	378,235	82,957
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	148,965	166,861
Gross Unrealized Gains	961	445
Gross Unrealized Losses	(343)	(760)
Securities available-for-sale	149,583	166,546
Collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	382,935	136,777
Gross Unrealized Gains	260	23
Gross Unrealized Losses	(1,152)	(924)
Securities available-for-sale	382,043	135,876
Private label collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	93,011	110,891
Gross Unrealized Gains	4,141	6,076
Gross Unrealized Losses	(230)	(21)
Securities available-for-sale	96,922	116,946
SBA asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	34,388	39,437
Gross Unrealized Gains	1,659	2,217
Gross Unrealized Losses	(19)	(27)
Securities available-for-sale	36,028	41,627
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	496,749	308,803
Gross Unrealized Gains	12,457	3,490
Gross Unrealized Losses	(498)	(404)
Securities available-for-sale	508,708	311,889
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Total securities available-for-sale	873,480	4,989
Gross Unrealized Gains	4,266	313
Gross Unrealized Losses	(593)	0
Securities available-for-sale	$ 877,153	$ 5,302