Investment Securities Schedule of Amortized Cost and Fair Values of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Statement [Line Items]
Amortized Cost	$ 2,947,500
Gross Unrealized Gains	119,921
Gross Unrealized Losses	(5,278)
Estimated Fair Value	3,062,143	$ 2,497,537
Amortized Cost, Held to Maturity	278,281	369,841
Gross Unrealized Gains, Held to Maturity	18,576
Gross Unrealized Losses, Held to Maturity	0
HTM securities	296,857
State and municipal securities
Statement [Line Items]
Amortized Cost	891,327
Gross Unrealized Gains	61,286
Gross Unrealized Losses	0
Estimated Fair Value	952,613
Amortized Cost, Held to Maturity		369,841
Gross Unrealized Gains, Held to Maturity		13,864
Gross Unrealized Losses, Held to Maturity		0
HTM securities		383,705
Amortized Cost		638,125
Gross Unrealized Gains		15,826
Gross Unrealized Losses		(1,024)
Estimated Fair Value		652,927
Corporate debt securities
Statement [Line Items]
Amortized Cost	348,391
Gross Unrealized Gains	19,445
Gross Unrealized Losses	(691)
Estimated Fair Value	367,145
Amortized Cost		370,401
Gross Unrealized Gains		8,490
Gross Unrealized Losses		(1,534)
Estimated Fair Value		377,357
Collateralized mortgage obligations
Statement [Line Items]
Amortized Cost	491,321
Gross Unrealized Gains	12,560
Gross Unrealized Losses	(115)
Estimated Fair Value	503,766
Amortized Cost		682,307
Gross Unrealized Gains		11,726
Gross Unrealized Losses		(315)
Estimated Fair Value		693,718
Residential mortgage-backed securities
Statement [Line Items]
Amortized Cost	373,779
Gross Unrealized Gains	4,246
Gross Unrealized Losses	(27)
Estimated Fair Value	377,998
Amortized Cost, Held to Maturity	278,281	369,841
Gross Unrealized Gains, Held to Maturity	18,576	13,864
Gross Unrealized Losses, Held to Maturity	0	0
HTM securities	296,857	383,705
Amortized Cost		177,183
Gross Unrealized Gains		1,078
Gross Unrealized Losses		(949)
Estimated Fair Value		177,312
Commercial mortgage-backed securities
Statement [Line Items]
Amortized Cost	741,172
Gross Unrealized Gains	22,384
Gross Unrealized Losses	(1,141)
Estimated Fair Value	762,415
Amortized Cost		489,603
Gross Unrealized Gains		6,471
Gross Unrealized Losses		(1,777)
Estimated Fair Value		494,297
Auction rate securities
Statement [Line Items]
Amortized Cost	101,510
Gross Unrealized Gains	0
Gross Unrealized Losses	(3,304)
Estimated Fair Value	$ 98,206
Amortized Cost		107,410
Gross Unrealized Gains		0
Gross Unrealized Losses		(5,484)
Estimated Fair Value		101,926
Debt securities
Statement [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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