Securities (Tables)

3 Months Ended

Mar. 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):

	March 31, 2024
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	445,399	$	—	$	(13,231	)	$	432,168
Obligations of states and political subdivisions		1,592,523		570		(139,058	)		1,454,035
Residential mortgage-backed securities		2,685,896		72		(386,691	)		2,299,277
Commercial mortgage-backed securities		381,198		52		(12,344	)		368,906
Corporate bonds and other		112,501		—		(8,361	)		104,140
Total securities available-for-sale	$	5,217,517	$	694	$	(559,685	)	$	4,658,526

	March 31, 2023
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	508,692	$	32	$	(20,482	)	$	488,242
Obligations of states and political subdivisions		1,935,401		1,562		(163,351	)		1,773,612
Residential mortgage-backed securities		2,957,314		17		(374,161	)		2,583,170
Commercial mortgage-backed securities		364,956		—		(15,020	)		349,936
Corporate bonds and other		112,677		—		(9,080	)		103,597
Total securities available-for-sale	$	5,879,040	$	1,611	$	(582,094	)	$	5,298,557

	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 March 31, 2024, by contractual and expected maturity, are shown below (dollars in thousands):

	Amortized		Estimated
	Cost Basis		Fair Value
Due within one year	$	343,112	$	337,823
Due after one year through five years		1,374,546		1,288,485
Due after five years through ten years		2,421,721		2,126,343
Due after ten years		1,078,138		905,875
Total	$	5,217,517	$	4,658,526

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

The following tables disclose as of March 31, 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
March 31, 2024	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
U.S. Treasury securities	$	2,438	$	16	$	429,730	$	13,215	$	432,168	$	13,231
Obligations of states and political subdivisions		33,073		114		1,398,108		138,944	$	1,431,181		139,058
Residential mortgage-backed securities		22,586		299		2,261,311		386,392	$	2,283,897		386,691
Commercial mortgage-backed securities		64,721		507		275,924		11,837	$	340,645		12,344
Corporate bonds and other		—		—		104,140		8,361	$	104,140		8,361
Total	$	122,818	$	936	$	4,469,213	$	558,749	$	4,592,031	$	559,685

	Less than 12 Months				12 Months or Longer				Total
March 31, 2023	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
U.S. Treasury securities	$	191,725	$	4,193	$	292,587	$	16,289	$	484,312	$	20,482
Obligations of states and political subdivisions		126,532		1,448		1,470,950		161,903		1,597,482		163,351
Residential mortgage-backed securities		91,738		2,937		2,489,524		371,224		2,581,262		374,161
Commercial mortgage-backed securities		164,301		3,948		185,635		11,072		349,936		15,020
Corporate bonds and other		42,503		1,167		61,094		7,913		103,597		9,080
Total	$	616,799	$	13,693	$	4,499,790	$	568,401	$	5,116,589	$	582,094

	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