Securities (Tables)

9 Months Ended

Sep. 30, 2022

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, 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

	September 30, 2021
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
Obligations of states and political subdivisions	$	2,568,224	$	103,646	$	(6,503	)	$	2,665,367
Residential mortgage-backed securities		2,999,369		35,950		(7,170	)		3,028,149
Commercial mortgage-backed securities		375,614		13,402		—			389,016
Corporate bonds and other		38,167		70		(785	)		37,452
Total securities available-for-sale	$	5,981,374	$	153,068	$	(14,458	)	$	6,119,984

	December 31, 2021
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	126,716	$	125	$	—		$	126,841
Obligations of states and political subdivisions		2,638,369		116,319		(1,217	)		2,753,471
Residential mortgage-backed securities		3,256,746		23,990		(21,287	)		3,259,449
Commercial mortgage-backed securities		356,207		8,914		(1	)		365,120
Corporate bonds and other		69,472		32		(1,206	)		68,298
Total securities available-for-sale	$	6,447,510	$	149,380	$	(23,711	)	$	6,573,179

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, 2022, by contractual and expected maturity, are shown below (dollars in thousands):

	Amortized		Estimated
	Cost Basis		Fair Value
Due within one year	$	239,974	$	238,511
Due after one year through five years		2,213,010		2,056,408
Due after five years through ten years		2,394,729		2,051,847
Due after ten years		1,698,758		1,398,677
Total	$	6,546,471	$	5,745,443

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

The following tables disclose as of September 30, 2022 and 2021, and December 31, 2021, 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, 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
September 30, 2021	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Obligations of states and political subdivisions	$	570,916	$	6,036	$	18,711	$	467	$	589,627	$	6,503
Residential mortgage-backed securities		1,208,129		7,166		2,051		4		1,210,180		7,170
Corporate bonds and other		32,985		785		—		—		32,985		785
Total	$	1,812,030	$	13,987	$	20,762	$	471	$	1,832,792	$	14,458

	Less than 12 Months				12 Months or Longer				Total
December 31, 2021	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Obligations of states and political subdivisions	$	163,698	$	1,096	$	18,943	$	122	$	182,641	$	1,218
Residential mortgage-backed securities		2,263,010		19,742		54,392		1,544		2,317,402		21,286
Commercial mortgage-backed securities		820		1		—		—		820		1
Corporate bonds and other		47,436		635		16,432		571		63,868		1,206
Total	$	2,474,964	$	21,474	$	89,767	$	2,237	$	2,564,731	$	23,711