Securities (Tables)

12 Months Ended

Dec. 31, 2021

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, 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

	December 31, 2020
	Amortized Cost Basis		Gross Unrealized Holding Gains		Gross Unrealized Holding Losses			Estimated Fair Value
Securities available-for-sale:
Obligations of state and political subdivisions	$	2,283,616	$	143,339	$	(79	)	$	2,426,876
Residential mortgage-backed securities		1,421,922		50,473		(115	)		1,472,280
Commercial mortgage-backed securities		467,243		22,077		(4	)		489,316
Corporate bonds and other		4,398		159		—			4,557
Total securities available-for-sale	$	4,177,179	$	216,048	$	(198	)	$	4,393,029

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, 2021, by contractual maturity, are shown below (in thousands):

	Amortized Cost Basis		Estimated Fair Value
Due within one year	$	261,110	$	264,173
Due after one year through five years		3,087,180		3,150,010
Due after five years through ten years		2,771,160		2,831,581
Due after ten years		328,060		327,415
Total	$	6,447,510	$	6,573,179

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

The following tables disclose, as of December 31, 2021 and 2020, 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, 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

	Less than 12 Months				12 Months or Longer				Total
December 31, 2020	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Obligations of state and political    subdivisions	$	25,214	$	79	$	—	$	—	$	25,214	$	79
Residential mortgage-backed securities		36,017		96		3,156		19		39,173		115
Commercial mortgage-backed securities		16,218		4		—		—		16,218		4
Total	$	77,449	$	179	$	3,156	$	19	$	80,605	$	198