Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Schedule of Investments [Line Items]
Amortized Cost	$ 2,735,991
Gross Unrealized Gains	77,413
Gross Unrealized Losses	(22,570)
Estimated Fair Value	2,790,834	$ 2,497,537
Amortized Cost, Held to Maturity	350,606	369,841
HTM securities	372,273
US States and Political Subdivisions Debt Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost	825,863
Gross Unrealized Gains	23,790
Gross Unrealized Losses	(1,569)
Estimated Fair Value	848,084
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	375,141
Gross Unrealized Gains	5,378
Gross Unrealized Losses	(7,041)
Estimated Fair Value	373,478
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	638,458
Gross Unrealized Gains	29,194
Gross Unrealized Losses	0
Estimated Fair Value	667,652
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	214,259
Gross Unrealized Gains	6,370
Gross Unrealized Losses	0
Estimated Fair Value	220,629
Amortized Cost, Held to Maturity	350,606	369,841
Gross Unrealized Gains, Held to Maturity	21,667	13,864
Gross Unrealized Losses, Held to Maturity	0	0
HTM securities	372,273	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	574,860
Gross Unrealized Gains	12,681
Gross Unrealized Losses	(216)
Estimated Fair Value	587,325
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	(13,744)
Estimated Fair Value	$ 93,666
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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