The amortized cost, gross unrealized gains and losses and estimated fair value of securities at June 30, 2021 and December 31, 2020 are summarized as follows (in thousands):


	June 30, 2021
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Allowance for Credit Losses		Fair Value
Available-for-Sale:
U.S. Government and agency obligations	$	94,847	$	1,039	$	(784)	$	—	$	95,102
Municipal bonds	311,533		17,180		(47)		—		328,666
Corporate bonds	268,387		3,703		(14)		—		272,076
Mortgage-backed or related securities	2,496,181		33,492		(28,176)		—		2,501,497
Asset-backed securities	78,631		7		—		—		78,638

	$	3,249,579	$	55,421	$	(29,021)	$	—	$	3,275,979


	June 30, 2021
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value		Allowance for Credit Losses
Held-to-Maturity:
U.S. Government and agency obligations	$	318	$	3	$	—	$	321	$	—
Municipal bonds	395,683		22,945		(803)		417,825		(64)
Corporate bonds	3,157		—		(7)		3,150		(46)
Mortgage-backed or related securities	56,208		2,473		—		58,681		—
	$	455,366	$	25,421	$	(810)	$	479,977	$	(110)


	December 31, 2020
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Allowance for Credit Losses		Fair Value
Available-for-Sale:
U.S. Government and agency obligations	$	141,668	$	1,002	$	(935)	$	—	$	141,735
Municipal bonds	283,997		19,523		(2)		—		303,518
Corporate bonds	219,086		2,762		(79)		—		221,769
Mortgage-backed or related securities	1,602,033		45,179		(1,060)		—		1,646,152
Asset-backed securities	9,405		77		(63)		—		9,419

	$	2,256,189	$	68,543	$	(2,139)	$	—	$	2,322,593


	December 31, 2020
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value		Allowance for Credit Losses
Held-to-Maturity:
U.S. Government and agency obligations	$	340	$	7	$	—	$	347	$	—
Municipal bonds	370,998		24,130		(94)		395,034		(59)
Corporate bonds	3,222		—		(12)		3,210		(35)
Mortgage-backed or related securities	47,247		2,843		—		50,090		—
	$	421,807	$	26,980	$	(106)	$	448,681	$	(94)

Accrued interest receivable on held-to-maturity debt securities was $3.0 million at both June 30, 2021 and December 31, 2020, and was $8.8 million and $6.9 million on available-for-sale debt securities as of June 30, 2021 and December 31, 2020, respectively. Accrued interest receivable on securities is reported in accrued interest receivable on the Consolidated Statements of Financial Condition and is excluded from the calculation of the allowance for credit losses.

At June 30, 2021 and December 31, 2020, the gross unrealized losses and the fair value for securities available-for-sale aggregated by the length of time that individual securities have been in a continuous unrealized loss position were as follows (in thousands):


	June 30, 2021
	Less Than 12 Months				12 Months or More				Total
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
Available-for-Sale:
U.S. Government and agency obligations	$	—	$	—	$	70,401	$	(784)	$	70,401	$	(784)



Municipal bonds	5,024		(40)		2,716		(7)		7,740		(47)
Corporate bonds	—		—		2,986		(14)		2,986		(14)





Mortgage-backed or related securities	1,102,279		(24,266)		203,238		(3,910)		1,305,517		(28,176)





	$	1,107,303	$	(24,306)	$	279,341	$	(4,715)	$	1,386,644	$	(29,021)


	December 31, 2020
	Less Than 12 Months				12 Months or More				Total
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
Available-for-Sale:
U.S. Government and agency obligations	$	3,126	$	(8)	$	50,603	$	(927)	$	53,729	$	(935)



Municipal bonds	495		(2)		—		—		495		(2)
Corporate bonds	3,586		(79)		—		—		3,586		(79)




Mortgage-backed or related securities	181,871		(1,046)		2,337		(14)		184,208		(1,060)



Asset-backed securities	—		—		5,676		(63)		5,676		(63)

	$	189,078	$	(1,135)	$	58,616	$	(1,004)	$	247,694	$	(2,139)

At June 30, 2021, there were 62 securities—available-for-sale with unrealized losses, compared to 54 at December 31, 2020. Management does not believe that any individual unrealized loss as of June 30, 2021 or December 31, 2020 resulted from credit loss. The decline in fair market value of these securities was generally due to changes in interest rates and changes in market-desired spreads subsequent to their purchase.

There were no sales of securities—trading during the six months ended June 30, 2021 or 2020. There were no securities—trading in a nonaccrual status at June 30, 2021 or December 31, 2020. Net unrealized holding gains of $117,000 were recognized during the six months ended June 30, 2021 compared to $2.4 million of net unrealized holding losses recognized during the six months ended June 30, 2020.


	Three Months Ended June 30,				Six Months Ended June 30,
	2021		2020		2021		2020
Available-for-Sale:
Gross Gains	$	77	$	93	$	626	$	171
Gross Losses	—		—		(110)		—
Balance, end of the period	$	77	$	93	$	516	$	171

There were no sales of securities—held-to-maturity during the six months ended June 30, 2021 or 2020. There were no securities—held-to-maturity in a nonaccrual status or 30 days or more past due at June 30, 2021 or December 31, 2020.

During the six months ended June 30, 2021, we sold a $4.8 million equity security with a resulting net gain of $46,000. There were no sales of equity securities during the six months ended June 30, 2020.

The amortized cost and estimated fair value of securities at June 30, 2021, by contractual maturity, are shown below (in thousands). Expected maturities will differ from contractual maturities because some securities may be called or prepaid with or without call or prepayment penalties.


	June 30, 2021
	Trading				Available-for-Sale				Held-to-Maturity
	Amortized Cost		Fair Value		Amortized Cost		Fair Value		Amortized Cost		Fair Value
Maturing in one year or less	$	—	$	—	$	163,091	$	163,421	$	5,085	$	5,125
Maturing after one year through five years	—		—		180,657		189,261		60,870		63,365
Maturing after five years through ten years	—		—		717,085		721,323		23,764		25,531
Maturing after ten years through twenty years	27,203		25,097		398,925		414,207		167,344		173,728
Maturing after twenty years	—		—		1,789,821		1,787,767		198,303		212,228


	$	27,203	$	25,097	$	3,249,579	$	3,275,979	$	455,366	$	479,977

The following table presents, as of June 30, 2021, investment securities which were pledged to secure borrowings, public deposits or other obligations as permitted or required by law (in thousands):


	June 30, 2021
	Carrying Value		Amortized Cost		Fair Value
Purpose or beneficiary:
State and local governments public deposits	$	203,479	$	202,652	$	216,996
Interest rate swap counterparties	27,056		26,296		27,209
Repurchase agreements	282,227		286,539		282,227
Other	2,569		2,569		2,625
Total pledged securities	$	515,331	$	518,056	$	529,057

The Company monitors the credit quality of held-to-maturity debt securities through the use of credit rating. Credit ratings are reviewed and updated quarterly. The Company's non-rated held-to-maturity debt securities are primarily United States government sponsored enterprise debentures carrying minimal to no credit risk. The balance is local municipal debt from within the Company's footprint and is monitored through quarterly or annual financial review. This municipal debt is predominately essential service or unlimited general obligation backed debt. The following table summarizes the amortized cost of held-to-maturity debt securities by credit rating at June 30, 2021 and December 31, 2020 (in thousands):


	June 30, 2021
	U.S. Government and agency obligations		Municipal bonds		Corporate bonds		Mortgage-backed or related securities		Total
AAA/AA/A	$	—	$	361,917	$	500	$	—	$	362,417

Not Rated	318		33,766		2,657		56,208		92,949


	$	318	$	395,683	$	3,157	$	56,208	$	455,366


	December 31, 2020
	U.S. Government and agency obligations		Municipal bonds		Corporate bonds		Mortgage-backed or related securities		Total
AAA/AA/A	$	—	$	349,123	$	500	$	—	$	349,623

Not Rated	340		21,875		2,722		47,247		72,184
	$	340	$	370,998	$	3,222	$	47,247	$	421,807

The following table presents the activity in the allowance for credit losses for held-to-maturity debt securities by major type for the three and six months ended June 30, 2021 and June 30, 2020 (in thousands):