Securities (Securities Available for Sale) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Proceeds from Sale of Debt Securities, Available-for-sale	$ 0	$ 175,586	$ 194,105	$ 259,554
Debt securities available-for-sale, amortized cost	3,491,307		3,491,307		$ 3,703,096
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	203,971		203,971		63,486
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(1,491)		(1,491)		(20,440)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0		0
Debt securities available for sale at fair value	3,693,787		3,693,787		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	2,698,160		2,698,160		2,864,949
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	168,231		168,231		47,223
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(1,361)		(1,361)		(19,222)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0		0
Debt securities available for sale at fair value	2,865,030		2,865,030		2,892,950
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	215,620		215,620		194,563
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	8,729		8,729		2,476
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(73)		(73)		(989)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0		0
Debt securities available for sale at fair value	224,276		224,276		196,050
State and Municipal Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	448,923		448,923		478,366
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	19,749		19,749		10,660
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(57)		(57)		(224)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0		0
Debt securities available for sale at fair value	468,615		468,615		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	128,604		128,604		165,218
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	7,262		7,262		3,127
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	0		0		(5)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0		0
Debt securities available for sale at fair value	$ 135,866		$ 135,866		$ 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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