Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Jan. 01, 2022	Dec. 31, 2021
AFS investment securities:
Amortized Cost	$ 3,394,924		$ 4,278,589
Gross Unrealized Gain	2,367		46,232
Gross Unrealized Loss	(175,196)		(50,957)
Estimated Fair Value	3,222,095		4,273,864
HTM investment securities:
Amortized Cost	996,423		381,696
Allowance for Credit Losses	(41)		(22)
Net Carrying Amount	996,382		381,674
Gross Unrecognized Gain	96		4,398
Gross Unrecognized Loss	(110,428)		(1,649)
Estimated Fair Value	886,050		384,423
U.S. Treasury
AFS investment securities:
Amortized Cost	14,930		57,708
Gross Unrealized Gain	0		614
Gross Unrealized Loss	(940)		(456)
Estimated Fair Value	13,990		57,866
Agency
AFS investment securities:
Amortized Cost	406,654		440,183
Gross Unrealized Gain	0		2,081
Gross Unrealized Loss	(30,877)		(10,129)
Estimated Fair Value	375,777		432,135
Corporate debt
AFS investment securities:
Amortized Cost	612,647		451,621
Gross Unrealized Gain	702		6,096
Gross Unrealized Loss	(18,284)		(3,856)
Estimated Fair Value	595,065		453,861
Municipal bonds
AFS investment securities:
Amortized Cost	445,537		1,061,985
Gross Unrealized Gain	58		32,209
Gross Unrealized Loss	(30,311)		(4,281)
Estimated Fair Value	415,284		1,089,913
HTM investment securities:
Amortized Cost	747,887		368,344
Allowance for Credit Losses	(41)		(22)
Net Carrying Amount	747,846		368,322
Gross Unrecognized Gain	0		3,834
Gross Unrecognized Loss	(101,757)	$ (6,900)	(1,649)
Estimated Fair Value	646,089		370,507
Collateralized mortgage obligations
AFS investment securities:
Amortized Cost	874,382		680,686
Gross Unrealized Gain	1,606		2,012
Gross Unrealized Loss	(20,732)		(6,055)
Estimated Fair Value	855,256		676,643
Mortgage-backed securities
AFS investment securities:
Amortized Cost	1,040,774		1,586,406
Gross Unrealized Gain	1		3,220
Gross Unrealized Loss	(74,052)		(26,180)
Estimated Fair Value	966,723		1,563,446
HTM investment securities:
Amortized Cost	247,050		11,843
Allowance for Credit Losses	0		0
Net Carrying Amount	247,050		11,843
Gross Unrecognized Gain	96		564
Gross Unrecognized Loss	(8,671)	$ (16,200)	0
Estimated Fair Value	238,475		12,407
Other
HTM investment securities:
Amortized Cost	1,486		1,509
Allowance for Credit Losses	0		0
Net Carrying Amount	1,486		1,509
Gross Unrecognized Gain	0		0
Gross Unrecognized Loss	0		0
Estimated Fair Value	$ 1,486		$ 1,509

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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