Investment Securities (Tables)

12 Months Ended

Dec. 31, 2021

Investments, Debt and Equity Securities [Abstract]

Amortized Cost and Estimated Fair Values of Investments Securities

The amortized cost and estimated fair values of investment securities classified as available for sale and held to maturity are summarized in the following tables:

	December 31, 2021
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
			(in thousands)
Debt Securities Available for Sale
Obligations of U.S. government agencies	$	1,260,226	$	8,193	$	(11,030)	$	1,257,389
Obligations of states and political subdivisions	187,197		5,832		(785)		192,244
Corporate bonds	6,722		34		—		6,756
Asset backed securities	754,185		2,354		(4,990)		751,549
Total debt securities available for sale	$	2,208,330	$	16,413	$	(16,805)	$	2,207,938
Debt Securities Held to Maturity
Obligations of U.S. government agencies	192,068		8,131		—		200,199
Obligations of states and political subdivisions	7,691		250		—		7,941
Total debt securities held to maturity	$	199,759	$	8,381	$	—	$	208,140

	December 31, 2020
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
			(in thousands)
Debt Securities Available for Sale
Obligations of U.S. government agencies	$	795,555	$	17,710	$	(891)	$	812,374
Obligations of states and political subdivisions	123,347		5,748		—		129,095
Corporate bonds	2,459		85		—		2,544
Asset backed securities	473,720		1,682		(5,151)		470,251
Total debt securities available for sale	$	1,395,081	$	25,225	$	(6,042)	$	1,414,264
Debt Securities Held to Maturity
Obligations of U.S. government agencies	$	273,667	$	13,774	$	—	$	287,441
Obligations of states and political subdivisions	10,896		389		—		11,285
Total debt securities held to maturity	$	284,563	$	14,163	$	—	$	298,726

Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity

Debt Securities	Available for Sale				Held to Maturity
(In thousands)	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Due in one year	$	7,601	$	7,645	$	—	$	—
Due after one year through five years	131,657		130,955		1,766		1,890
Due after five years through ten years	341,925		342,606		16,612		17,090
Due after ten years	1,727,147		1,726,732		181,381		189,160
Totals	$	2,208,330	$	2,207,938	$	199,759	$	208,140

Gross Unrealized Losses on Investment Securities

Gross unrealized losses on investment securities and the fair value of the related securities, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, were as follows:

	Less than 12 months				12 months or more				Total
	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
December 31, 2021	(in thousands)
Debt Securities Available for Sale
Obligations of U.S. government agencies	$	947,108	$	(9,737)	$	44,086	$	(1,293)	$	991,194	$	(11,030)
Obligations of states and political subdivisions	56,153		(785)		—		—		56,153		(785)
Asset backed securities	389,837		(4,118)		109,748		(872)		499,585		(4,990)
Total debt securities available for sale	$	1,393,098	$	(14,640)	$	153,834	$	(2,165)	$	1,546,932	$	(16,805)

	Less than 12 months				12 months or more				Total
	Fair Value		Unrealized Loss		Fair Value		Unrealized Loss		Fair Value		Unrealized Loss
December 31, 2020	(in thousands)
Debt Securities Available for Sale
Obligations of U.S. government agencies	$	160,543	$	(891)	$	—	$	—	$	160,543	$	(891)
Obligations of states and political subdivisions	—		—		—		—		—		—
Asset backed securities	51,544		(441)		297,020		(4,710)		348,564		(5,151)
Total securities available for sale	$	212,087	$	(1,332)	$	297,020	$	(4,710)	$	509,107	$	(6,042)