The following table summarizes the amortized cost and fair value of available-for-sale debt securities by major type:


	At December 31, 2021
(In thousands)	Amortized Cost (1)		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value (2)
U.S. Treasury notes	$	398,664	$	—	$	(1,698)	$	396,966
Agency CMO	88,109		2,326		(51)		90,384
Agency MBS	1,568,293		36,130		(11,020)		1,593,403
Agency CMBS	1,248,548		2,537		(18,544)		1,232,541
CMBS	887,640		506		(1,883)		886,263
CLO	21,860		—		(13)		21,847
Corporate debt	14,583		—		(1,133)		13,450
Available-for-sale debt securities	$	4,227,697	$	41,499	$	(34,342)	$	4,234,854

	At December 31, 2020
(In thousands)	Amortized Cost (1)		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value (2)
Agency CMO	$	148,711	$	6,000	$	(98)	$	154,613
Agency MBS	1,389,100		68,598		(289)		1,457,409
Agency CMBS	1,092,430		26,317		(1,514)		1,117,233
CMBS	512,759		1,082		(5,823)		508,018
CLO	76,693		—		(310)		76,383
Corporate debt	14,557		—		(1,437)		13,120
Available-for-sale debt securities	$	3,234,250	$	101,997	$	(9,471)	$	3,326,776

(1)Accrued interest receivable of $7.5 million at both December 31, 2021 and 2020 is excluded from amortized cost and is reported within accrued interest receivable and other assets on the accompanying Consolidated Balance Sheets.

The following table summarizes the gross unrealized losses and fair value of available-for-sale debt securities by length of time each major security type has been in a continuous unrealized loss position and for which an ACL has not been recorded:


	At December 31, 2021
	Less Than 12 Months				12 Months or More				Total
(Dollars in thousands)	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		# of Holdings	Fair Value		Unrealized Losses
U.S. Treasury notes	$	396,966	$	(1,698)	$	—	$	—	8	$	396,966	$	(1,698)
Agency CMO	7,895		(51)		—		—		2	7,895		(51)
Agency MBS	506,602		(7,354)		110,687		(3,666)		70	617,289		(11,020)
Agency CMBS	632,213		(6,163)		335,480		(12,381)		28	967,693		(18,544)
CMBS	724,762		(1,744)		81,253		(139)		50	806,015		(1,883)
CLO	—		—		21,848		(13)		1	21,848		(13)
Corporate debt	4,203		(76)		9,247		(1,057)		3	13,450		(1,133)
Available-for-sale debt securities in unrealized loss position	$	2,272,641	$	(17,086)	$	558,515	$	(17,256)	162	$	2,831,156	$	(34,342)


	At December 31, 2020
	Less Than 12 Months				12 Months or More				Total
(Dollars in thousands)	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		# of Holdings	Fair Value		Unrealized Losses
Agency CMO	$	13,137	$	(49)	$	5,944	$	(49)	5	$	19,081	$	(98)
Agency MBS	33,742		(219)		4,561		(70)		30	38,303		(289)
Agency CMBS	376,330		(1,514)		—		—		8	376,330		(1,514)
CMBS	409,591		(5,486)		23,167		(337)		38	432,758		(5,823)
CLO	57,728		(265)		18,655		(45)		4	76,383		(310)
Corporate debt	4,100		(166)		9,020		(1,271)		3	13,120		(1,437)
Available-for-sale debt securities in unrealized loss position	$	894,628	$	(7,699)	$	61,347	$	(1,772)	88	$	955,975	$	(9,471)

Available-for-sale debt securities in a continuous unrealized loss position have been assessed for impairment. Since the Company does not intend to sell nor will it be required to sell these securities prior to their anticipated recovery, and as the securities are investment grade, management concluded that no impairment is required. The increase in unrealized losses from 2020 to 2021 is primarily due to portfolio activity and higher market rates. Market prices will approach par as the securities approach maturity. There were no available-for-sale debt securities in non-accrual status at December 31, 2021 and 2020.

The following table summarizes the amortized cost and fair value of available-for-sale debt securities by contractual maturity:


	At December 31, 2021
(In thousands)	Amortized Cost		Fair Value
Maturing within 1 year	$	—	$	—
After 1 year through 5 years	401,668		400,037
After 5 years through 10 years	113,288		113,260
After 10 years	3,712,741		3,721,557
Available-for-sale debt securities	$	4,227,697	$	4,234,854

Available-for-sale debt securities that are not due at a single maturity date have been categorized based on the maturity date of the underlying collateral. Actual principal cash flows may differ from this categorization as borrowers have the right to prepay their obligations with or without prepayment penalties.

There were no sales of available-for-sale debt securities during the year ended December 31, 2021. During the year ended December 31, 2020, the Company sold available-for-sale debt securities for cash proceeds of $9.0 million, which resulted in gross realized gains of $8 thousand. During the year ended December 31, 2019, the Company sold available-for-sale debt securities for cash proceeds of $70.1 million, which resulted in gross realized gains of $773 thousand and gross realized losses of $744 thousand ($29 thousand net gain on sale).


	At December 31, 2021
(In thousands)	Amortized Cost (1)		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value		Allowance for Credit Losses		Net Carrying Value
Agency CMO	$	42,405	$	655	$	(25)	$	43,035	$	—	$	42,405
Agency MBS	2,901,593		71,444		(11,788)		2,961,249		—		2,901,593
Agency CMBS	2,378,475		11,202		(43,844)		2,345,833		—		2,378,475
Municipal bonds and notes	705,918		51,572		—		757,490		214		705,704
CMBS	169,948		3,381		—		173,329		—		169,948
Held-to-maturity debt securities	$	6,198,339	$	138,254	$	(55,657)	$	6,280,936	$	214	$	6,198,125

	At December 31, 2020
(In thousands)	Amortized Cost (1)		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value		Allowance for Credit Losses		Net Carrying Value
Agency CMO	$	91,622	$	1,785	$	(241)	$	93,166	$	—	$	91,622
Agency MBS	2,419,751		137,863		(84)		2,557,530		—		2,419,751
Agency CMBS	2,101,227		60,484		(2,213)		2,159,498		—		2,101,227
Municipal bonds and notes	739,507		60,371		(3)		799,875		299		739,208
CMBS	216,081		9,214		—		225,295		—		216,081
Held-to-maturity debt securities	$	5,568,188	$	269,717	$	(2,541)	$	5,835,364	$	299	$	5,567,889

(1)Accrued interest receivable of $21.2 million and $22.1 million at December 31, 2021 and December 31, 2020, respectively, is excluded from amortized cost and is reported within accrued interest receivable and other assets on the accompanying Consolidated Balance Sheets.

An ACL on held-to-maturity debt securities is recorded for certain municipal bonds and notes to account for expected lifetime credit losses. Agency securities represent obligations issued by a U.S. government-sponsored enterprise or other federally-related entity, and are either explicitly or implicitly guaranteed and therefore, assumed to be zero loss. Held-to-maturity debt securities with gross unrealized losses and no ACL are considered to be of high credit quality and therefore, zero credit loss is recorded as of December 31, 2021. The current period unrealized loss position of certain Agency CMBS is primarily attributed to the changing interest rate environment.

The following table summarizes the activity in the ACL on held-to-maturity debt securities:


	Years ended December 31,
(In thousands)	2021		2020
Balance, beginning of period (1)	$	299	$	—
Adoption of CECL	—		397
(Benefit) for credit losses	(85)		(98)
Balance, end of period	$	214	$	299


	At December 31, 2021
(In thousands)	Amortized Cost		Fair Value
Maturing within 1 year	$	180	$	180
After 1 year through 5 years	7,425		7,712
After 5 years through 10 years	292,727		304,190
After 10 years	5,898,007		5,968,854
Held-to-maturity debt securities	$	6,198,339	$	6,280,936

Held-to-maturity debt securities that are not due at a single maturity date have been categorized based on the maturity date of the underlying collateral. Actual principal cash flows may differ from this categorization as borrowers have the right to prepay their obligations with or without prepayment penalties.

The Company monitors the credit quality of held-to-maturity debt securities through credit ratings provided by Standard & Poor's Rating Services (S&P), Moody's Investor Services (Moody's), Fitch Ratings Inc., Kroll Bond Rating Agency, and DBRS Inc. Credit ratings express opinions about the credit quality of a debt security, and are updated at each quarter end. Investment grade debt securities are rated BBB- or higher by S&P, or Baa3 or higher by Moody's, and are generally considered by the rating agencies and market participants to be of low credit risk. Conversely, debt securities rated below investment grade, which are labeled as speculative grade by the rating agencies, are considered to have distinctively higher credit risk than investment grade debt securities. There were no speculative grade held-to-maturity debt securities at December 31, 2021 and 2020. Held-to-maturity debt securities that are not rated are collateralized with U.S. Treasury obligations.


	At December 31, 2021
	Investment Grade
(In thousands)	Aaa		Aa1		Aa2		Aa3		A1		A2		A3		Baa2		Not Rated
Agency CMOs	$	—	$	42,405	$	—	$	—	$	—	$	—	$	—	$	—	$	—
Agency MBS	—		2,901,593		—		—		—		—		—		—		—
Agency CMBS	—		2,378,475		—		—		—		—		—		—		—
Municipal bonds and notes	207,426		119,804		227,106		104,232		35,878		8,260		—		95		3,117
CMBS	169,948		—		—		—		—		—		—		—		—
Held-to-maturity debt securities	$	377,374	$	5,442,277	$	227,106	$	104,232	$	35,878	$	8,260	$	—	$	95	$	3,117


	At December 31, 2020
	Investment Grade
(In thousands)	Aaa		Aa1		Aa2		Aa3		A1		A2		A3		Baa2		Not Rated
Agency CMOs	$	—	$	91,622	$	—	$	—	$	—	$	—	$	—	$	—	$	—
Agency MBS	—		2,419,751		—		—		—		—		—		—		—
Agency CMBS	—		2,101,227		—		—		—		—		—		—		—
Municipal bonds and notes	209,376		165,056		201,081		115,619		33,264		8,475		2,066		190		4,380
CMBS	216,081		—		—		—		—		—		—		—		—
Held-to-maturity debt securities	$	425,457	$	4,777,656	$	201,081	$	115,619	$	33,264	$	8,475	$	2,066	$	190	$	4,380

At December 31, 2021 and 2020, there were no held-to-maturity debt securities past due under the terms of their agreements or in non-accrual status.

At December 31, 2021, Webster had callable CMBS, collateralized loan obligation securities (CLO), corporate debt, and municipal bonds and notes with an aggregate carrying value of $1.6 billion. The Company considers this prepayment risk in the evaluation of its interest rate risk profile.

Available-for-sale and held-to-maturity debt securities with carrying values of $1.8 billion and $3.1 billion, respectively, at December 31, 2021, and $1.3 billion and $2.6 billion, respectively, at December 31, 2020, were pledged to secure public funds, trust deposits, repurchase agreements, and other purposes, as required or permitted by law.