Securities (Debt securities) (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Debt Securities, [Line Items]
Debt securities available-for-sale, amortized cost	$ 4,103,196	$ 4,997,529
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	103,979	219,873
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(17,109)	(7,268)
Debt securities available for sale at fair value	4,190,066	5,210,134
Debt Securities, Held-to-Maturity, Amortized Cost	2,024,715	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	9,586
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	1,321
Debt Securities, Held-to-maturity, Fair Value	2,032,980	0
U.S. Government Agency and Government-Sponsored Enterprise Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member]
Debt Securities, [Line Items]
Debt securities available-for-sale, amortized cost	2,671,827	3,640,351
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	74,715	178,579
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(9,342)	(4,543)
Debt securities available for sale at fair value	2,737,200	3,814,387
Debt Securities, Held-to-Maturity, Amortized Cost	2,024,715
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	9,586
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	1,321
Debt Securities, Held-to-maturity, Fair Value	2,032,980
Asset-backed Securities [Member]
Debt Securities, [Line Items]
Debt securities available-for-sale, amortized cost	429,983	349,904
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	6,307	9,651
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(3,976)	(2,076)
Debt securities available for sale at fair value	432,314	357,479
State and Municipal Securities [Member]
Debt Securities, [Line Items]
Debt securities available-for-sale, amortized cost	748,224	729,066
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	18,294	25,098
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(2,458)	(592)
Debt securities available for sale at fair value	764,060	753,572
U.S. Government and Government-Sponsored Enterprise Securities [Member]
Debt Securities, [Line Items]
Debt securities available-for-sale, amortized cost	253,162	278,208
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4,663	6,545
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(1,333)	(57)
Debt securities available for sale at fair value	$ 256,492	$ 284,696

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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