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


	September 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	$	180,585	$	1,067	$	(600)	$	—	$	181,052
Municipal bonds	296,784		14,395		(788)		—		310,391
Corporate bonds	160,404		3,903		(63)		—		164,244
Mortgage-backed or related securities	2,599,229		25,918		(40,883)		—		2,584,264
Asset-backed securities	206,514		110		—		—		206,624

	$	3,443,516	$	45,393	$	(42,334)	$	—	$	3,446,575


	September 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	$	317	$	6	$	—	$	323	$	—
Municipal bonds	391,583		18,716		(2,188)		408,111		(63)
Corporate bonds	3,124		—		(2)		3,122		(41)
Mortgage-backed or related securities	52,788		1,668		(10)		54,446		—
	$	447,812	$	20,390	$	(2,200)	$	466,002	$	(104)


	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 $2.7 million and $3.0 million as of September 30, 2021 and December 31, 2020, respectively, and was $9.3 million and $6.9 million on available-for-sale debt securities as of September 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 September 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):


	September 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	$	40,115	$	(39)	$	33,447	$	(561)	$	73,562	$	(600)



Municipal bonds	60,522		(788)		—		—		60,522		(788)
Corporate bonds	7,937		(63)		—		—		7,937		(63)





Mortgage-backed or related securities	1,503,114		(39,701)		54,570		(1,182)		1,557,684		(40,883)





	$	1,611,688	$	(40,591)	$	88,017	$	(1,743)	$	1,699,705	$	(42,334)


	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 September 30, 2021, there were 84 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 September 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 nine months ended September 30, 2021 or 2020. There were no securities—trading in a nonaccrual status at September 30, 2021 or December 31, 2020. Net unrealized holding gains of $1.9 million were recognized during the nine months ended September 30, 2021 compared to $2.4 million of net unrealized holding losses recognized during the nine months ended September 30, 2020.


	Three Months Ended September 30,				Nine Months Ended September 30,
	2021		2020		2021		2020
Available-for-Sale:
Gross Gains	$	140	$	557	$	766	$	729
Gross Losses	(22)		(432)		(132)		(433)
Balance, end of the period	$	118	$	125	$	634	$	296

During the nine months ended September 30, 2021, we sold one held-to-maturity security with a resulting net gain of $3,000 and had partial calls of securities that resulted in a net loss of $65,000. There were no sales of securities—held-to-maturity during the nine months ended September 30, 2020. There were no securities—held-to-maturity in a nonaccrual status or 30 days or more past due at September 30, 2021 or December 31, 2020.

During the nine months ended September 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 nine months ended September 30, 2020. During the nine months ended September 30, 2020, the Company sold Visa Class B stock with a net gain of $519,000. The stock was previously carried at a zero-cost basis due to transfer restrictions and uncertainty of litigation.

The amortized cost and estimated fair value of securities at September 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.


	September 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	$	—	$	—	$	59,150	$	59,360	$	5,444	$	5,471
Maturing after one year through five years	—		—		173,666		181,152		59,705		62,039
Maturing after five years through ten years	—		—		845,638		847,708		21,961		23,312
Maturing after ten years through twenty years	27,203		26,875		502,832		514,271		164,303		168,315
Maturing after twenty years	—		—		1,862,230		1,844,084		196,399		206,865


	$	27,203	$	26,875	$	3,443,516	$	3,446,575	$	447,812	$	466,002

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


	September 30, 2021
	Carrying Value		Amortized Cost		Fair Value
Purpose or beneficiary:
State and local governments public deposits	$	202,085	$	201,456	$	213,687
Interest rate swap counterparties	27,275		26,642		27,402
Repurchase agreements	278,087		285,068		278,087
Other	2,549		2,549		2,597
Total pledged securities	$	509,996	$	515,715	$	521,773

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 geographic 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 September 30, 2021 and December 31, 2020 (in thousands):


	September 30, 2021
	U.S. Government and agency obligations		Municipal bonds		Corporate bonds		Mortgage-backed or related securities		Total
AAA/AA/A	$	—	$	375,865	$	500	$	—	$	376,365

Not Rated	317		15,718		2,624		52,788		71,447


	$	317	$	391,583	$	3,124	$	52,788	$	447,812


	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 nine months ended September 30, 2021 and September 30, 2020 (in thousands):