Securities (Securities Available for Sale) (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Proceeds from Sale of Debt Securities, Available-for-sale	$ 194,105	$ 83,968
Debt securities available-for-sale, amortized cost	3,406,492		$ 3,703,096
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	148,415		63,486
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(1,779)		(20,440)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0
Debt securities available for sale at fair value	3,553,128		3,746,142
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	2,643,659		2,864,949
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	123,926		47,223
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(1,448)		(19,222)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0
Debt securities available for sale at fair value	2,766,137		2,892,950
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	196,250		194,563
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	7,309		2,476
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(47)		(989)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0
Debt securities available for sale at fair value	203,512		196,050
State and Municipal Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	437,957		478,366
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	10,387		10,660
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(284)		(224)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0
Debt securities available for sale at fair value	448,060		488,802
U.S. Government and Government-Sponsored Enterprise Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	128,626		165,218
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	6,793		3,127
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0		(5)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0
Debt securities available for sale at fair value	$ 135,419		$ 168,340

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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