Securities (Tables)

9 Months Ended

Sep. 30, 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 (in thousands):

	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

	September 30, 2020
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
Obligations of states and political subdivisions	$	2,252,412	$	119,952	$	(4,875	)	$	2,367,489
Residential mortgage-backed securities		1,454,422		54,744		(112	)		1,509,054
Commercial mortgage-backed securities		527,392		22,776		—			550,168
Corporate bonds and other		4,398		171		—			4,569
Total securities available-for-sale	$	4,238,624	$	197,643	$	(4,987	)	$	4,431,280

	December 31, 2020
			Gross		Gross
	Amortized		Unrealized		Unrealized			Estimated
	Cost Basis		Holding Gains		Holding Losses			Fair Value
Securities available-for-sale:
Obligations of states and political subdivisions	$	2,283,616	$	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 September 30, 2021, by contractual and expected maturity, are shown below (in thousands):

	Amortized		Estimated
	Cost Basis		Fair Value
Due within one year	$	473,997	$	478,547
Due after one year through five years		2,934,396		3,023,175
Due after five years through ten years		2,485,328		2,531,243
Due after ten years		87,653		87,019
Total	$	5,981,374	$	6,119,984

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

The following tables disclose 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
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
September 30, 2020	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
Obligations of states and political subdivisions	$	395,729	$	4,875	$	—	$	—	$	395,729	$	4,875
Residential mortgage-backed securities		49,559		76		4,916		36		54,475		112
Total	$	445,288	$	4,951	$	4,916	$	36	$	450,204	$	4,987

	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