Securities (Tables)

12 Months Ended

Dec. 31, 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):

	December 31, 2024
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	278,155	$	2	$	(4,595	)	$	273,562
Obligations of state and political subdivisions		1,578,749		338		(138,842	)		1,440,245
Residential mortgage-backed securities		2,729,096		184		(378,746	)		2,350,534
Commercial mortgage-backed securities		457,311		99		(9,658	)		447,752
Corporate bonds and other		111,994		50		(6,378	)		105,666
Total securities available-for-sale	$	5,155,305	$	673	$	(538,219	)	$	4,617,759

	December 31, 2023
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	496,975	$	4	$	(14,745	)	$	482,234
Obligations of state 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 December 31, 2024, by contractual maturity, are shown below (in thousands):

	Amortized Cost Basis		Estimated Fair Value
Due within one year	$	292,438	$	288,985
Due after one year through five years		1,815,382		1,678,422
Due after five years through ten years		2,073,461		1,821,254
Due after ten years		974,024		829,098
Total	$	5,155,305	$	4,617,759

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

The following tables disclose, as of December 31, 2024 and 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 (in thousands):

	Less than 12 Months					12 Months or Longer					Total
December 31, 2024	Fair Value		Unrealized Loss			Fair Value		Unrealized Loss			Fair Value		Unrealized Loss
U.S. Treasury Securities	$	—	$	—		$	271,088	$	(4,595	)	$	271,088	$	(4,595	)
Obligations of state and political    subdivisions		56,777		(1,224	)		1,361,628		(137,618	)		1,418,405		(138,842	)
Residential mortgage-backed securities		198,385		(4,990	)		2,048,727		(373,756	)		2,247,112		(378,746	)
Commercial mortgage-backed securities		196,640		(2,249	)		190,189		(7,409	)		386,829		(9,658	)
Corporate bonds and other		4,871		(44	)		95,840		(6,334	)		100,711		(6,378	)
Total	$	456,673	$	(8,507	)	$	3,967,472	$	(529,712	)	$	4,424,145	$	(538,219	)

	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 state 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	)