Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Amortized Cost	$ 2,548,261
Gross Unrealized Gains	105,138
Gross Unrealized Losses	(9,100)
Estimated Fair Value	2,644,299	$ 2,497,537
Amortized Cost, Held to Maturity	330,514	369,841
HTM securities	354,109
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	852,617
Gross Unrealized Gains	41,292
Gross Unrealized Losses	(227)
Estimated Fair Value	893,682
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	312,507
Gross Unrealized Gains	15,168
Gross Unrealized Losses	(2,322)
Estimated Fair Value	325,353
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	559,281
Gross Unrealized Gains	19,113
Gross Unrealized Losses	0
Estimated Fair Value	578,394
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	156,152
Gross Unrealized Gains	4,716
Gross Unrealized Losses	0
Estimated Fair Value	160,868
Amortized Cost, Held to Maturity	330,514	369,841
Gross Unrealized Gains, Held to Maturity	23,595	13,864
Gross Unrealized Losses, Held to Maturity	0	0
HTM securities	354,109	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	560,294
Gross Unrealized Gains	24,849
Gross Unrealized Losses	0
Estimated Fair Value	585,143
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	107,410
Gross Unrealized Gains	0
Gross Unrealized Losses	(6,551)
Estimated Fair Value	$ 100,859
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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