Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Jul. 01, 2021	Dec. 31, 2020
Investment securities available-for-sale:
Amortized Cost	$ 4,705,112		$ 3,846,733
Gross Unrealized Gain	47,186		90,004
Gross Unrealized Loss	(42,483)		(5,622)
Estimated Fair Value	4,709,815		3,931,115
Investment securities held-to-maturity:
Amortized Cost	170,587		23,732
Allowance for Credit Losses	(11)		0
Net Carrying Amount	170,576		23,732
Gross Unrecognized Gain	803		1,281
Gross Unrecognized Loss	(2,225)		0
Estimated Fair Value	169,154		25,013
U.S. Treasury
Investment securities available-for-sale:
Amortized Cost	77,803		30,153
Gross Unrealized Gain	1,665		2,380
Gross Unrealized Loss	(126)		0
Estimated Fair Value	79,342		32,533
Agency
Investment securities available-for-sale:
Amortized Cost	531,867		666,702
Gross Unrealized Gain	5,252		24,292
Gross Unrealized Loss	(7,026)		(608)
Estimated Fair Value	530,093		690,386
Corporate
Investment securities available-for-sale:
Amortized Cost	383,910		412,223
Gross Unrealized Gain	5,381		3,591
Gross Unrealized Loss	(2,900)		(506)
Estimated Fair Value	386,391		415,308
Municipal bonds
Investment securities available-for-sale:
Amortized Cost	1,237,923		1,412,012
Gross Unrealized Gain	27,585		37,260
Gross Unrealized Loss	(9,642)		(3,253)
Estimated Fair Value	1,255,866		1,446,019
Investment securities held-to-maturity:
Amortized Cost	154,399
Allowance for Credit Losses	(11)
Net Carrying Amount	154,388
Gross Unrecognized Gain	0
Gross Unrecognized Loss	(2,225)	$ (3,200)
Estimated Fair Value	152,163
Collateralized mortgage obligations
Investment securities available-for-sale:
Amortized Cost	665,851		513,259
Gross Unrealized Gain	1,109		819
Gross Unrealized Loss	(4,729)		(712)
Estimated Fair Value	662,231		513,366
Mortgage-backed securities
Investment securities available-for-sale:
Amortized Cost	1,807,758		812,384
Gross Unrealized Gain	6,194		21,662
Gross Unrealized Loss	(18,060)		(543)
Estimated Fair Value	1,795,892		833,503
Investment securities held-to-maturity:
Amortized Cost	14,656		22,124
Allowance for Credit Losses	0		0
Net Carrying Amount	14,656		22,124
Gross Unrecognized Gain	803		1,281
Gross Unrecognized Loss	0		0
Estimated Fair Value	15,459		23,405
Other
Investment securities held-to-maturity:
Amortized Cost	1,532		1,608
Allowance for Credit Losses	0		0
Net Carrying Amount	1,532		1,608
Gross Unrecognized Gain	0		0
Gross Unrecognized Loss	0		0
Estimated Fair Value	$ 1,532		$ 1,608

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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