Securities (Debt securities Available for Sale) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Proceeds from Sale of Debt Securities, Available-for-sale	$ 0	$ 0	$ 194,105	$ 259,554
Debt securities available-for-sale, amortized cost	4,081,118		4,081,118		$ 3,703,096
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	205,654		205,654		63,486
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(5,052)		(5,052)		(20,440)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0		0
Debt securities available for sale at fair value	4,281,720		4,281,720		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	2,957,028		2,957,028		2,864,949
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	168,271		168,271		47,223
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(3,657)		(3,657)		(19,222)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0		0
Debt securities available for sale at fair value	3,121,642		3,121,642		2,892,950
Asset-backed Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	284,499		284,499		194,563
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	9,055		9,055		2,476
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(530)		(530)		(989)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0		0
Debt securities available for sale at fair value	293,024		293,024		196,050
State and Municipal Securities [Member]
Debt Securities, Available-for-sale [Line Items]
Debt securities available-for-sale, amortized cost	536,861		536,861		478,366
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	21,396		21,396		10,660
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(813)		(813)		(224)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0		0
Debt securities available for sale at fair value	557,444		557,444		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	302,730		302,730		165,218
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax	6,932		6,932		3,127
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax	(52)		(52)		(5)
Debt Securities, Available-for-sale, Allowance for Credit Loss	0		0		0
Debt securities available for sale at fair value	$ 309,610		$ 309,610		$ 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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