Securities (Tables)

6 Months Ended

Jun. 30, 2024

Cash and Cash Equivalents [Abstract]

Summary of Available-for-Sale Securities

The amortized cost, related gross unrealized gains and losses, allowance for credit losses and the fair value of available-for-sale securities are as follows (dollars in thousands):

	June 30, 2024
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	352,682	$	—	$	(10,706	)	$	341,976
Obligations of states and political subdivisions		1,584,389		420		(139,436	)		1,445,373
Residential mortgage-backed securities		2,705,508		71		(389,494	)		2,316,085
Commercial mortgage-backed securities		362,004		—		(11,982	)		350,022
Corporate bonds and other		127,868		—		(8,300	)		119,568
Total securities available-for-sale	$	5,132,451	$	491	$	(559,918	)	$	4,573,024

	June 30, 2023
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	508,124	$	—	$	(24,348	)	$	483,776
Obligations of states and political subdivisions		1,859,193		405		(166,634	)		1,692,964
Residential mortgage-backed securities		2,875,699		2		(402,760	)		2,472,941
Commercial mortgage-backed securities		331,829		—		(17,442	)		314,387
Corporate bonds and other		112,507		—		(10,313	)		102,194
Total securities available-for-sale	$	5,687,352	$	407	$	(621,497	)	$	5,066,262

	December 31, 2023
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	496,975	$	4	$	(14,745	)	$	482,234
Obligations of states and political subdivisions		1,621,405		934		(125,182	)		1,497,157
Residential mortgage-backed securities		2,716,968		7		(352,883	)		2,364,092
Commercial mortgage-backed securities		295,663		—		(11,339	)		284,324
Corporate bonds and other		112,670		—		(7,715	)		104,955
Total securities available-for-sale	$	5,243,681	$	945	$	(511,864	)	$	4,732,762

Amortized Cost and Estimated Fair Value of Available-for-Sale Securities

The amortized cost and estimated fair value of available-for-sale securities at June 30, 2024, by contractual and expected maturity, are shown below (dollars in thousands):

	Amortized		Estimated
	Cost Basis		Fair Value
Due within one year	$	304,065	$	299,230
Due after one year through five years		1,709,307		1,576,132
Due after five years through ten years		2,143,482		1,856,004
Due after ten years		975,597		841,658
Total	$	5,132,451	$	4,573,024

Continuous Unrealized-Loss Position of Available-for-Sale Securities

The following tables disclose as of June 30, 2024 and 2023, and December 31, 2023, the Company’s investment securities that have been in a continuous unrealized-loss position for less than 12 months and for 12 or more months (dollars in thousands):

	Less than 12 Months				12 Months or Longer				Total
June 30, 2024	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
U.S. Treasury securities	$	1,448	$	12	$	340,528	$	10,694	$	341,976	$	10,706
Obligations of states and political subdivisions		32,423		320		1,391,496		139,117	$	1,423,919		139,437
Residential mortgage-backed securities		80,096		462		2,183,346		389,031	$	2,263,442		389,493
Commercial mortgage-backed securities		92,487		928		257,535		11,054	$	350,022		11,982
Corporate bonds and other		—		—		104,031		8,300	$	104,031		8,300
Total	$	206,454	$	1,722	$	4,276,936	$	558,196	$	4,483,390	$	559,918

	Less than 12 Months				12 Months or Longer				Total
June 30, 2023	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
U.S. Treasury securities	$	47,498	$	1,614	$	436,278	$	22,734	$	483,776	$	24,348
Obligations of states and political subdivisions		137,728		1,838		1,457,431		164,796		1,595,159		166,634
Residential mortgage-backed securities		68,439		3,609		2,403,837		399,151		2,472,276		402,760
Commercial mortgage-backed securities		99,347		4,771		215,040		12,671		314,387		17,442
Corporate bonds and other		38,042		1,282		64,152		9,031		102,194		10,313
Total	$	391,054	$	13,114	$	4,576,738	$	608,383	$	4,967,792	$	621,497

	Less than 12 Months				12 Months or Longer				Total
December 31, 2023	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
U.S. Treasury securities	$	3,477	$	7	$	477,306	$	14,738	$	480,783	$	14,745
Obligations of states and political subdivisions		11,855		34		1,427,975		125,148		1,439,830		125,182
Residential mortgage-backed securities		1,631		1		2,361,089		352,882		2,362,720		352,883
Commercial mortgage-backed securities		—		—		284,324		11,339		284,324		11,339
Corporate bonds and other		—		—		104,955		7,715		104,955		7,715
Total	$	16,963	$	42	$	4,655,649	$	511,822	$	4,672,612	$	511,864