Securities (Tables)

6 Months Ended

Jun. 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):

	June 30, 2022
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	501,098	$	12	$	(14,752	)	$	486,358
Obligations of states and political subdivisions		2,419,232		7,737		(182,365	)		2,244,604
Residential mortgage-backed securities		3,265,654		399		(301,149	)		2,964,904
Commercial mortgage-backed securities		423,168		476		(8,819	)		414,825
Corporate bonds and other		113,184		88		(8,927	)		104,345
Total securities available-for-sale	$	6,722,336	$	8,712	$	(516,012	)	$	6,215,036

	June 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	$	2,490,468	$	125,612	$	(993	)	$	2,615,087
Residential mortgage-backed securities		2,471,278		38,619		(5,711	)		2,504,186
Commercial mortgage-backed securities		405,146		15,822		—			420,968
Corporate bonds and other		38,297		91		(581	)		37,807
Total securities available-for-sale	$	5,405,189	$	180,144	$	(7,285	)	$	5,578,048

	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 June 30, 2022, by contractual and expected maturity, are shown below (dollars in thousands):

	Amortized		Estimated
	Cost Basis		Fair Value
Due within one year	$	239,670	$	239,640
Due after one year through five years		2,912,498		2,780,193
Due after five years through ten years		2,925,448		2,630,741
Due after ten years		644,720		564,462
Total	$	6,722,336	$	6,215,036

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

The following tables disclose as of June 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
June 30, 2022	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
U.S. Treasury securities	$	466,438	$	14,752	$	—	$	—	$	466,438	$	14,752
Obligations of states and political subdivisions		1,623,146		178,446		24,038		3,919		1,647,184		182,365
Residential mortgage-backed securities		2,631,511		266,352		277,404		34,797		2,908,915		301,149
Commercial mortgage-backed securities		341,997		8,819		—		—		341,997		8,819
Corporate bonds and other		73,358		4,287		26,397		4,640		99,755		8,927
Total	$	5,136,450	$	472,656	$	327,839	$	43,356	$	5,464,289	$	516,012

	Less than 12 Months				12 Months or Longer				Total
June 30, 2021	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Obligations of states and political subdivisions	$	170,483	$	992	$	—	$	—	$	170,483	$	992
Residential mortgage-backed securities		712,100		5,706		2,478		6		714,578		5,712
Commercial mortgage-backed securities		866		—		—		—		866		—
Corporate bonds and other		33,318		581		—		—		33,318		581
Total	$	916,767	$	7,279	$	2,478	$	6	$	919,245	$	7,285

	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