Securities (Tables)

9 Months Ended

Sep. 30, 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):

	September 30, 2023
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	507,549	$	—	$	(22,102	)	$	485,447
Obligations of states and political subdivisions		1,726,947		120		(263,313	)		1,463,754
Residential mortgage-backed securities		2,792,813		—		(495,578	)		2,297,235
Commercial mortgage-backed securities		323,539		—		(18,191	)		305,348
Corporate bonds and other		112,338		—		(11,585	)		100,753
Total securities available-for-sale	$	5,463,186	$	120	$	(810,769	)	$	4,652,537

	September 30, 2022
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	504,457	$	2	$	(27,283	)	$	477,176
Obligations of states and political subdivisions		2,368,605		514		(285,307	)		2,083,812
Residential mortgage-backed securities		3,153,864		14		(456,997	)		2,696,881
Commercial mortgage-backed securities		406,530		1		(19,185	)		387,346
Corporate bonds and other		113,015		-		(12,787	)		100,228
Total securities available-for-sale	$	6,546,471	$	531	$	(801,559	)	$	5,745,443

	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 September 30, 2023, by contractual and expected maturity, are shown below (dollars in thousands):

	Amortized		Estimated
	Cost Basis		Fair Value
Due within one year	$	335,508	$	327,912
Due after one year through five years		1,345,049		1,214,288
Due after five years through ten years		2,466,782		2,034,867
Due after ten years		1,315,847		1,075,470
Total	$	5,463,186	$	4,652,537

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

The following tables disclose as of September 30, 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
September 30, 2023	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
U.S. Treasury securities	$	4,867	$	49	$	480,580	$	22,053	$	485,447	$	22,102
Obligations of states and political subdivisions		53,029		2,568		1,402,097		260,745	$	1,455,126		263,313
Residential mortgage-backed securities		3,123		106		2,294,112		495,472	$	2,297,235		495,578
Commercial mortgage-backed securities		1,274		87		304,074		18,104	$	305,348		18,191
Corporate bonds and other		—		—		100,753		11,585	$	100,753		11,585
Total	$	62,293	$	2,810	$	4,581,616	$	807,959	$	4,643,909	$	810,769

	Less than 12 Months				12 Months or Longer				Total
September 30, 2022	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
U.S. Treasury securities	$	476,696	$	27,283	$	—	$	—	$	476,696	$	27,283
Obligations of states and political subdivisions		1,828,105		254,212		139,732		31,095		1,967,837		285,307
Residential mortgage-backed securities		1,507,121		231,915		1,187,815		225,082		2,694,936		456,997
Commercial mortgage-backed securities		386,479		19,185		—		—		386,479		19,185
Corporate bonds and other		59,782		4,544		40,446		8,243		100,228		12,787
Total	$	4,258,183	$	537,139	$	1,367,993	$	264,420	$	5,626,176	$	801,559

	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