Securities (Debt securities) (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt Securities, [Line Items]
Debt securities available-for-sale, amortized cost	$ 4,773,742	$ 4,997,529
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	88,414	219,873
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(30,237)	(7,268)
Debt securities available for sale at fair value	4,831,919	5,210,134
Debt Securities, Held-to-Maturity, Amortized Cost	2,075,158	0
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	2,052
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	6,159
Debt Securities, Held-to-maturity, Fair Value	2,071,051	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	3,333,804	3,640,351
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	63,072	178,579
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(18,015)	(4,543)
Debt securities available for sale at fair value	3,378,861	3,814,387
Debt Securities, Held-to-Maturity, Amortized Cost	2,075,158
Debt Securities, Held-to-maturity, Accumulated Unrecognized Gain	2,052
Debt Securities, Held-to-maturity, Accumulated Unrecognized Loss	6,159
Debt Securities, Held-to-maturity, Fair Value	2,071,051
Asset-backed Securities [Member]
Debt Securities, [Line Items]
Debt securities available-for-sale, amortized cost	417,128	349,904
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4,803	9,651
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(6,022)	(2,076)
Debt securities available for sale at fair value	415,909	357,479
State and Municipal Securities [Member]
Debt Securities, [Line Items]
Debt securities available-for-sale, amortized cost	769,775	729,066
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	16,370	25,098
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(4,972)	(592)
Debt securities available for sale at fair value	781,173	753,572
U.S. Government and Government-Sponsored Enterprise Securities [Member]
Debt Securities, [Line Items]
Debt securities available-for-sale, amortized cost	253,035	278,208
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	4,169	6,545
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(1,228)	(57)
Debt securities available for sale at fair value	$ 255,976	$ 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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