Securities (Tables)

3 Months Ended

Mar. 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):

	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

	March 31, 2022
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
U.S. Treasury securities		311,860		-		(10,516	)		301,344
Obligations of states and political subdivisions		2,590,308		26,628		(87,455	)		2,529,481
Residential mortgage-backed securities		3,450,006		2,139		(186,954	)		3,265,191
Commercial mortgage-backed securities		341,506		924		(4,752	)		337,678
Corporate bonds and other		74,287		-		(5,486	)		68,801
Total securities available-for-sale	$	6,767,967	$	29,691	$	(295,163	)	$	6,502,495

	December 31, 2022
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	508,275	$	11	$	(25,737	)	$	482,549
Obligations of states 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 March 31, 2023, by contractual and expected maturity, are shown below (dollars in thousands):

	Amortized		Estimated
	Cost Basis		Fair Value
Due within one year	$	267,375	$	264,338
Due after one year through five years		1,499,938		1,420,069
Due after five years through ten years		3,126,228		2,771,041
Due after ten years		985,499		843,109
Total	$	5,879,040	$	5,298,557

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

The following tables disclose as of March 31, 2023 and 2022, and December 31, 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 (dollars in thousands):

	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
March 31, 2022	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
U.S. Treasury securities	$	301,344	$	10,516	$	—	$	—	$	301,344	$	10,516
Obligations of states and political subdivisions		1,518,732		85,015		25,679		2,440		1,544,411		87,455
Residential mortgage-backed securities		2,725,934		163,034		303,344		23,920		3,029,278		186,954
Commercial mortgage-backed securities		202,719		4,752		—		—		202,719		4,752
Corporate bonds and other		40,701		2,431		28,100		3,055		68,801		5,486
Total	$	4,789,430	$	265,748	$	357,123	$	29,415	$	5,146,553	$	295,163

	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 states 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