The following tables set forth the amortized cost, gross unrealized gains and losses, and estimated fair value of debt securities classified as available for sale and held to maturity at December 31, 2020 and 2019. Amortized cost of securities does not include accrued interest which is reflected in the accrued interest line item on the consolidated balance sheets totaling $24.4 million and $23.9 million at December 31, 2020 and December 31, 2019, respectively.

Securities Available for Sale
	December 31, 2020								December 31, 2019
			Gross		Gross						Gross		Gross
	Amortized		Unrealized		Unrealized		Fair		Amortized		Unrealized		Unrealized		Fair
(in thousands)	Cost		Gains		Losses		Value		Cost		Gains		Losses		Value
U.S. Treasury and government agency securities	$	207,365	$	6,289	$	284	$	213,370	$	98,320	$	652	$	300	$	98,672
Municipal obligations		309,342		17,536		153		326,725		242,016		7,789		—		249,805
Residential mortgage-backed securities		2,560,249		69,570		8		2,629,811		1,910,909		20,268		7,020		1,924,157
Commercial mortgage-backed securities		2,323,306		135,516		3,288		2,455,534		1,570,765		19,880		4,178		1,586,467
Collateralized mortgage obligations		354,472		7,651		—		362,123		807,600		3,757		3,142		808,215
Corporate debt securities		11,500		264		—		11,764		8,000		21		33		7,988
	$	5,766,234	$	236,826	$	3,733	$	5,999,327	$	4,637,610	$	52,367	$	14,673	$	4,675,304

Securities Held to Maturity
	December 31, 2020								December 31, 2019
			Gross		Gross						Gross		Gross
	Amortized		Unrealized		Unrealized		Fair		Amortized		Unrealized		Unrealized		Fair
(in thousands)	Cost		Gains		Losses		Value		Cost		Gains		Losses		Value
U.S. Treasury and government agency securities	$	—	$	—	$	—	$	—	$	50,000	$	3	$	—	$	50,003
Municipal obligations		627,019		51,408		2		678,425		641,019		27,146		69		668,096
Residential mortgage-backed securities		21,951		1,469		—		23,420		29,687		883		—		30,570
Commercial mortgage-backed securities		549,686		54,587		—		604,273		539,371		12,474		581		551,264
Collateralized mortgage obligations		158,514		2,949		—		161,463		307,932		3,597		458		311,071
	$	1,357,170	$	110,413	$	2	$	1,467,581	$	1,568,009	$	44,103	$	1,108	$	1,611,004

The Company held no securities classified as trading at December 31, 2020 or 2019.

The following tables present the amortized cost and fair value of debt securities at December 31, 2020 by contractual maturity. Actual maturities will differ from contractual maturities because of rights to call or repay obligations with or without penalties and scheduled and unscheduled principal payments on mortgage-backed securities and collateral mortgage obligations.

The following table presents the proceeds from, gross gains on, and gross losses on sales of securities during the years ended December 31, 2020, 2019 and 2018:

Securities with carrying values totaling approximately $3.4 billion at December 31, 2020 and $3.3 billion at December 31, 2019 were pledged, primarily to secure public deposits or securities sold under agreements to repurchase.

The Company’s policy is to invest only in securities of investment grade quality. These investments are largely limited to U.S. agency securities and municipal securities. Management has concluded, based on the long history of no credit losses, that the expectation of nonpayment of the held to maturity securities carried at amortized cost is zero for securities that are backed by the full faith and credit of and/or guaranteed by the U.S. government. As such, no allowance for credit losses has been recorded for these securities. The municipal portfolio is analyzed separately for allowance for credit loss in accordance with the applicable guidance for each portfolio as noted below.

Effective January 1, 2020, in conjunction with the adoption of CECL, and again at the end of each reporting period, the Company evaluated credit impairment for individual securities available for sale whose fair value was below amortized cost with a more than inconsequential risk of default and where the Company had assessed the decline in fair value significant enough to suggest a credit event occurred. There were no securities that met the criteria of a credit loss event and therefore, no allowance for credit loss was recorded in any period.

The details for securities classified as available for sale with unrealized losses at December 31, 2020 follow.

Available for sale
	Losses < 12 Months				Losses 12 Months or >				Total
			Gross				Gross				Gross
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
(in thousands)	Value		Losses		Value		Losses		Value		Losses
U.S. Treasury and government agency securities	$	35,845	$	284	$	—	$	—	$	35,845	$	284
Municipal obligations		30,170		153		—		—		30,170		153
Residential mortgage-backed securities		530		2		760		6		1,290		8
Commercial mortgage-backed securities		446,190		3,288		—		—		446,190		3,288
Collateralized mortgage obligations		70		—		—		—		70		—
Corporate debt securities		2,000		—		—		—		2,000		—
.	$	514,805	$	3,727	$	760	$	6	$	515,565	$	3,733

The details for securities classified as available for sale with unrealized losses at December 31, 2019 follow.

Available for sale
	Losses < 12 Months				Losses 12 Months or >				Total
			Gross				Gross				Gross
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
(in thousands)	Value		Losses		Value		Losses		Value		Losses
U.S. Treasury and government agency securities	$	28,235	$	300	$	—	$	—	$	28,235	$	300
Municipal obligations		—		—		—		—		—		—
Residential mortgage-backed securities		420,066		5,042		399,787		1,978		819,853		7,020
Commercial mortgage-backed securities		458,855		3,971		14,896		207		473,751		4,178
Collateralized mortgage obligations		89,689		1,315		184,389		1,827		274,078		3,142
Corporate debt securities		1,467		33		—		—		1,467		33
	$	998,312	$	10,661	$	599,072	$	4,012	$	1,597,384	$	14,673

Effective January 1, 2020 and in conjunction with the adoption of CECL, and again at the end of each reporting period, the Company evaluated its held to maturity municipal obligation portfolio for credit loss using probability of default and loss given default models. The models were run using a long-term average probability of default migration and with a probability weighting of Moody’s economic forecasts. The economic forecasts were largely weighted to a baseline scenario with some weight given to other scenarios. The December 31, 2020 forecast was further stressed by running a more severe probability of default migration. The resulting credit loss, if any, were negligible and no allowance for credit loss was recorded.

The fair value and gross unrealized losses for securities classified as held to maturity with unrealized losses at December 31, 2020 follow.

Held to maturity
	Losses < 12 Months				Losses 12 Months or >				Total
			Gross				Gross				Gross
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
(in thousands)	Value		Losses		Value		Losses		Value		Losses
U.S. Treasury and government agency securities	$	—	$	—	$	—	$	—	$	—	$	—
Municipal obligations		—		—		2,381		2		2,381		2
Residential mortgage-backed securities		—		—		—		—		—		—
Commercial mortgage-backed securities		—		—		—		—		—		—
Collateralized mortgage obligations		—		—		—		—		—		—
	$	—	$	—	$	2,381	$	2	$	2,381	$	2

The fair value and gross unrealized losses for securities classified as held to maturity with unrealized losses at December 31, 2019 follow.

Held to maturity
	Losses < 12 Months				Losses 12 Months or >				Total
			Gross				Gross				Gross
	Fair		Unrealized		Fair		Unrealized		Fair		Unrealized
(in thousands)	Value		Losses		Value		Losses		Value		Losses
U.S. Treasury and government agency securities	$	—	$	—	$	—	$	—	$	—	$	—
Municipal obligations		4,735		38		3,143		31		7,878		69
Residential mortgage-backed securities		—		—		—		—		—		—
Commercial mortgage-backed securities		28,426		581		—		—		28,426		581
Collateralized mortgage obligations		—		—		49,110		458		49,110		458
	$	33,161	$	619	$	52,253	$	489	$	85,414	$	1,108

At December 31, 2020 and 2019, the Company had 28 and 155 securities, respectively, with market values below their cost basis. None of the unrealized losses relate to the marketability of the securities or the issuers’ abilities to meet contractual obligations. In all cases, the indicated impairment on these debt securities would be recovered no later than the security’s maturity date or possibly earlier if the market price for the security increases with a reduction in the yield required by the market. The unrealized losses were been deemed to be non-credit related at December 31, 2020 and 2019. As noted above, no allowance for credit loss was recorded as of January 1, 2020 or December 31, 2020. The Company has adequate liquidity and, therefore, does not plan to and, more likely than not, will not be required to sell these securities before recovery of the indicated impairment.