Securities

Schedule of amortized cost and fair value of debt securities available for sale and held to maturity by contractual maturity

All mortgage-backed securities were issued by U.S. government corporations and agencies (dollars in thousands):


	As of September 30, 2021
	Amortized		Fair
	Cost		Value
Debt securities available for sale
Due in one year or less	$	103,158	$	103,787
Due after one year through five years		603,868		608,074
Due after five years through ten years		385,479		393,540
Due after ten years		2,904,919		2,891,843
Total debt securities available for sale	$	3,997,424	$	3,997,244

Schedule of securities with gross unrealized losses aggregated by investment category and length of time that individual securities have been in a continuous loss position

The following information pertains to debt securities with gross unrealized losses, aggregated by investment category and the length of time that individual securities have been in a continuous loss position (dollars in thousands):


	As of September 30, 2021
	Less than 12 months				12 months or more				Total
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
	Value		Losses		Value		Losses		Value		Losses
Debt securities available for sale
U.S. Treasury securities	$	180,317	$	(255)	$	—	$	—	$	180,317	$	(255)
Obligations of states and political subdivisions		55,434		(754)		—		—		55,434		(754)
Commercial mortgage-backed securities		434,220		(9,036)		3,063		(42)		437,283		(9,078)
Residential mortgage-backed securities		1,392,160		(19,707)		5,568		(46)		1,397,728		(19,753)
Asset-backed securities		51,183		(134)		—		—		51,183		(134)
Corporate debt securities		210,951		(1,182)		—		—		210,951		(1,182)
Total temporarily impaired securities	$	2,324,265	$	(31,068)	$	8,631	$	(88)	$	2,332,896	$	(31,156)


	As of December 31, 2020
	Less than 12 months				12 months or more				Total
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
	Value		Losses		Value		Losses		Value		Losses
Debt securities available for sale
Obligations of U.S. government corporations and agencies	$	—	$	—	$	4,957	$	(49)	$	4,957	$	(49)
Obligations of states and political subdivisions		762		(8)		—		—		762		(8)
Commercial mortgage-backed securities		129,655		(312)		—		—		129,655		(312)
Residential mortgage-backed securities		89,997		(300)		139		(3)		90,136		(303)
Corporate debt securities		1,499		(1)		—		—		1,499		(1)
Total temporarily impaired securities	$	221,913	$	(621)	$	5,096	$	(52)	$	227,009	$	(673)