Securities (Tables)

12 Months Ended

Dec. 31, 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):

	December 31, 2022
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	508,275	$	11	$	(25,737	)	$	482,549
Obligations of state 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

	December 31, 2021
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value
Securities available-for-sale:
U.S. Treasury securities	$	126,716	$	125	$	—		$	126,841
Obligations of state 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 December 31, 2022, by contractual maturity, are shown below (in thousands):

	Amortized Cost Basis		Estimated Fair Value
Due within one year	$	210,053	$	207,169
Due after one year through five years		1,741,024		1,632,058
Due after five years through ten years		2,715,108		2,376,526
Due after ten years		1,486,163		1,258,606
Total	$	6,152,348	$	5,474,359

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

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

	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 state 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

	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 state 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