Investment Securities - Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	$ 40,767	$ 3,378
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	177,369	241,999
Debt Securities, Available-for-sale	9,276,926	5,235,591
Total securities available-for-sale	9,140,324	4,996,970
Government agency and government-sponsored enterprise collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	3,739	280
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	29,440	47,994
Debt Securities, Available-for-sale	1,088,537	1,219,880
Total securities available-for-sale	1,062,836	1,172,166
Municipal securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	6,991	18
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	67,167	93,631
Debt Securities, Available-for-sale	1,989,929	1,531,617
Total securities available-for-sale	1,929,753	1,438,004
Agency residential MBS
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	15,787	897
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	11,236	12,483
Debt Securities, Available-for-sale	2,180,993	341,074
Total securities available-for-sale	2,185,544	329,488
Private label commercial MBS
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	1,428	10
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	716	1,089
Debt Securities, Available-for-sale	388,318	82,957
Total securities available-for-sale	389,030	81,878
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	279	760
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	855	445
Debt Securities, Available-for-sale	132,997	166,546
Total securities available-for-sale	132,421	166,861
Collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	673	924
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	163	23
Debt Securities, Available-for-sale	358,547	135,876
Total securities available-for-sale	359,057	136,777
Private label collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	1,570	21
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	3,508	6,076
Debt Securities, Available-for-sale	303,681	116,946
Total securities available-for-sale	301,743	110,891
SBA asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	17	27
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	1,185	2,217
Debt Securities, Available-for-sale	32,017	41,627
Total securities available-for-sale	30,849	39,437
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	1,161	404
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	11,976	3,490
Debt Securities, Available-for-sale	516,615	311,889
Total securities available-for-sale	505,800	308,803
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	4,099	0
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	2,060	313
Debt Securities, Available-for-sale	971,268	5,302
Total securities available-for-sale	973,307	4,989
Government agency and government-sponsored enterprise commercial mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Debt Securities, Available-for-sale, Unrealized Loss	5,023	37
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	49,063	74,238
Debt Securities, Available-for-sale	1,314,024	1,281,877
Total securities available-for-sale	$ 1,269,984	$ 1,207,676

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of accumulated unrealized loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

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