Securities (Tables)

6 Months Ended

Jun. 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):

	June 30, 2023
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	508,124	$	—	$	(24,348	)	$	483,776
Obligations of states and political subdivisions		1,859,193		405		(166,634	)		1,692,964
Residential mortgage-backed securities		2,875,699		2		(402,760	)		2,472,941
Commercial mortgage-backed securities		331,829		—		(17,442	)		314,387
Corporate bonds and other		112,507		—		(10,313	)		102,194
Total securities available-for-sale	$	5,687,352	$	407	$	(621,497	)	$	5,066,262

	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

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

	Amortized		Estimated
	Cost Basis		Fair Value
Due within one year	$	326,804	$	320,734
Due after one year through five years		1,487,053		1,379,040
Due after five years through ten years		2,817,340		2,473,140
Due after ten years		1,056,155		893,348
Total	$	5,687,352	$	5,066,262

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

The following tables disclose as of June 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
June 30, 2023	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
U.S. Treasury securities	$	47,498	$	1,614	$	436,278	$	22,734	$	483,776	$	24,348
Obligations of states and political subdivisions		137,728		1,838		1,457,431		164,796		1,595,159		166,634
Residential mortgage-backed securities		68,439		3,609		2,403,837		399,151		2,472,276		402,760
Commercial mortgage-backed securities		99,347		4,771		215,040		12,671		314,387		17,442
Corporate bonds and other		38,042		1,282		64,152		9,031		102,194		10,313
Total	$	391,054	$	13,114	$	4,576,738	$	608,383	$	4,967,792	$	621,497

	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
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