Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Amortized Cost	$ 2,691,879
Gross Unrealized Gains	106,692
Gross Unrealized Losses	(5,091)
Estimated Fair Value	2,793,480	$ 2,497,537
Amortized Cost, Held to Maturity	304,241	369,841
HTM securities	324,940
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	880,495
Gross Unrealized Gains	46,216
Gross Unrealized Losses	(157)
Estimated Fair Value	926,554
Amortized Cost		638,125
Gross Unrealized Gains		15,826
Gross Unrealized Losses		(1,024)
Estimated Fair Value		652,927
Corporate Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	327,143
Gross Unrealized Gains	16,724
Gross Unrealized Losses	(1,309)
Estimated Fair Value	342,558
Amortized Cost		370,401
Gross Unrealized Gains		8,490
Gross Unrealized Losses		(1,534)
Estimated Fair Value		377,357
Collateralized Mortgage Obligations [Member]
Schedule of Investments [Line Items]
Amortized Cost	516,312
Gross Unrealized Gains	16,207
Gross Unrealized Losses	(46)
Estimated Fair Value	532,473
Amortized Cost		682,307
Gross Unrealized Gains		11,726
Gross Unrealized Losses		(315)
Estimated Fair Value		693,718
Mortgage-Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	318,343
Gross Unrealized Gains	3,409
Gross Unrealized Losses	(57)
Estimated Fair Value	321,695
Amortized Cost, Held to Maturity	304,241	369,841
Gross Unrealized Gains, Held to Maturity	20,699	13,864
Gross Unrealized Losses, Held to Maturity	0	0
HTM securities	324,940	383,705
Amortized Cost		177,183
Gross Unrealized Gains		1,078
Gross Unrealized Losses		(949)
Estimated Fair Value		177,312
Commercial Mortgage Backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	548,076
Gross Unrealized Gains	24,136
Gross Unrealized Losses	(2)
Estimated Fair Value	572,210
Amortized Cost		489,603
Gross Unrealized Gains		6,471
Gross Unrealized Losses		(1,777)
Estimated Fair Value		494,297
Auction Rate Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	101,510
Gross Unrealized Gains	0
Gross Unrealized Losses	(3,520)
Estimated Fair Value	$ 97,990
Amortized Cost		107,410
Gross Unrealized Gains		0
Gross Unrealized Losses		(5,484)
Estimated Fair Value		101,926
Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost		2,465,029
Gross Unrealized Gains		43,591
Gross Unrealized Losses		(11,083)
Estimated Fair Value		$ 2,497,537

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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