Securities (Tables)

12 Months Ended

Dec. 31, 2023

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, 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

	December 31, 2022
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	508,275	$	11	$	(25,737	)	$	482,549
Obligations of state and political subdivisions		2,104,193		1,217		(206,799	)		1,898,611
Residential mortgage-backed securities		3,034,120		8		(417,562	)		2,616,566
Commercial mortgage-backed securities		392,914		1		(18,046	)		374,869
Corporate bonds and other		112,846		—		(11,082	)		101,764
Total securities available-for-sale	$	6,152,348	$	1,237	$	(679,226	)	$	5,474,359

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, 2023, by contractual maturity, are shown below (in thousands):

	Amortized Cost Basis		Estimated Fair Value
Due within one year	$	360,853	$	355,802
Due after one year through five years		1,399,375		1,308,406
Due after five years through ten years		2,751,006		2,455,882
Due after ten years		732,447		612,672
Total	$	5,243,681	$	4,732,762

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

The following tables disclose, as of December 31, 2023 and 2022, 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, 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

	Less than 12 Months				12 Months or Longer				Total
December 31, 2022	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
U.S. Treasury Securities	$	307,012	$	11,650	$	173,105	$	14,087	$	480,117	$	25,737
Obligations of state and political    subdivisions		770,469		55,943		946,571		150,856		1,717,040		206,799
Residential mortgage-backed securities		470,970		37,065		2,143,869		380,497		2,614,839		417,562
Commercial mortgage-backed securities		319,303		11,677		54,862		6,369		374,165		18,046
Corporate bonds and other	$	41,920	$	1,698	$	59,844	$	9,384		101,764		11,082
Total	$	1,909,674	$	118,033	$	3,378,251	$	561,193	$	5,287,925	$	679,226