Securities (Debt securities Available for Sale) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	$ 5,417,373	$ 4,997,529
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	137,708	219,873
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(58,791)	(7,268)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt securities available for sale at fair value	5,496,290	5,210,134
U.S. Government Agency and Government-Sponsored Enterprise Mortgage-Backed Securities and Collateralized Mortgage Obligations [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	4,040,359	3,640,351
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	109,063	178,579
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(45,203)	(4,543)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt securities available for sale at fair value	4,104,219	3,814,387
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	359,259	349,904
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	5,216	9,651
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(5,656)	(2,076)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt securities available for sale at fair value	358,819	357,479
State and Municipal Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	739,570	729,066
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	18,080	25,098
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(6,365)	(592)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt securities available for sale at fair value	751,285	753,572
U.S. Government and Government-Sponsored Enterprise Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	278,185	278,208
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	5,349	6,545
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(1,567)	(57)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0	0
Debt securities available for sale at fair value	$ 281,967	$ 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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Amount, before tax, of unrealized loss 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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