Investment Securities - Amortized Cost and Estimated Fair Value of Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2021	Jul. 01, 2021	Dec. 31, 2020
Investment securities available-for-sale:
Amortized Cost	$ 4,278,589		$ 3,846,733
Gross Unrealized Gain	46,232		90,004
Gross Unrealized Loss	(50,957)		(5,622)
Estimated Fair Value	4,273,864		3,931,115
Investment securities held-to-maturity:
Amortized Cost	381,696		23,732
Allowance for Credit Losses	(22)		0
Net Carrying Amount	381,674		23,732
Gross Unrealized Gain	4,398		1,281
Gross Unrecognized Loss	(1,649)		0
Estimated Fair Value	384,423		25,013
U.S. Treasury
Investment securities available-for-sale:
Amortized Cost	57,708		30,153
Gross Unrealized Gain	614		2,380
Gross Unrealized Loss	(456)		0
Estimated Fair Value	57,866		32,533
Agency
Investment securities available-for-sale:
Amortized Cost	440,183		666,702
Gross Unrealized Gain	2,081		24,292
Gross Unrealized Loss	(10,129)		(608)
Estimated Fair Value	432,135		690,386
Corporate
Investment securities available-for-sale:
Amortized Cost	451,621		412,223
Gross Unrealized Gain	6,096		3,591
Gross Unrealized Loss	(3,856)		(506)
Estimated Fair Value	453,861		415,308
Municipal bonds
Investment securities available-for-sale:
Amortized Cost	1,061,985		1,412,012
Gross Unrealized Gain	32,209		37,260
Gross Unrealized Loss	(4,281)		(3,253)
Estimated Fair Value	1,089,913		1,446,019
Investment securities held-to-maturity:
Amortized Cost	368,344
Allowance for Credit Losses	(22)
Net Carrying Amount	368,322
Gross Unrealized Gain	3,834
Gross Unrecognized Loss	(1,649)	$ (6,600)
Estimated Fair Value	370,507
Collateralized mortgage obligations
Investment securities available-for-sale:
Amortized Cost	680,686		513,259
Gross Unrealized Gain	2,012		819
Gross Unrealized Loss	(6,055)		(712)
Estimated Fair Value	676,643		513,366
Mortgage-backed securities
Investment securities available-for-sale:
Amortized Cost	1,586,406		812,384
Gross Unrealized Gain	3,220		21,662
Gross Unrealized Loss	(26,180)		(543)
Estimated Fair Value	1,563,446		833,503
Investment securities held-to-maturity:
Amortized Cost	11,843		22,124
Allowance for Credit Losses	0		0
Net Carrying Amount	11,843		22,124
Gross Unrealized Gain	564		1,281
Gross Unrecognized Loss	0		0
Estimated Fair Value	12,407		23,405
Other
Investment securities held-to-maturity:
Amortized Cost	1,509		1,608
Allowance for Credit Losses	0		0
Net Carrying Amount	1,509		1,608
Gross Unrealized Gain	0		0
Gross Unrecognized Loss	0		0
Estimated Fair Value	$ 1,509		$ 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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