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 March 31, 2021 and December 31, 2020. 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 at both March 31, 2021 and December 31, 2020.

	March 31, 2021								December 31, 2020
			Gross		Gross				Gross		Gross
Securities Available for Sale	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	$	219,343	$	2,703	$	4,554	$	217,492	$	207,365	$	6,289	$	284	$	213,370
Municipal obligations		308,298		13,757		5,393		316,662		309,342		17,536		153		326,725
Residential mortgage-backed securities		2,976,621		45,861		51,672		2,970,810		2,560,249		69,570		8		2,629,811
Commercial mortgage-backed securities		2,789,053		78,043		64,901		2,802,195		2,323,306		135,516		3,288		2,455,534
Collateralized mortgage obligations		305,078		6,835		—		311,913		354,472		7,651		—		362,123
Corporate debt securities		11,500		76		—		11,576		11,500		264		—		11,764
	$	6,609,893	$	147,275	$	126,520	$	6,630,648	$	5,766,234	$	236,826	$	3,733	$	5,999,327

	March 31, 2021								December 31, 2020
			Gross		Gross				Gross		Gross
Securities Held to Maturity	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	$	—	$	—	$	—	$	—	$	—	$	—	$	—	$	—
Municipal obligations		624,649		42,623		311		666,961		627,019		51,408		2		678,425
Residential mortgage-backed securities		53,298		1,315		459		54,154		21,951		1,469		—		23,420
Commercial mortgage-backed securities		574,923		35,164		1,041		609,046		549,686		54,587		—		604,273
Collateralized mortgage obligations		122,472		2,591		—		125,063		158,514		2,949		—		161,463
	$	1,375,342	$	81,693	$	1,811	$	1,455,224	$	1,357,170	$	110,413	$	2	$	1,467,581

The following tables present the amortized cost and estimated fair value of debt securities available for sale and held to maturity at March 31, 2021 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 collateralized mortgage obligations.

The Company held no securities classified as trading at March 31, 2021 and December 31, 2020.

The following table presents the proceeds from, gross gain on, and gross losses on sales of securities during the three months ended March 31, 2021 and 2020.

Securities with carrying values totaling $3.0 billion and $3.4 billion were pledged as collateral at March 31, 2021 and December 31, 2020, respectively, 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.

At each reporting period, the Company evaluates 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 whether the decline in fair value was 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 for either period presented.

The fair value and gross unrealized losses for securities classified as available for sale with unrealized losses for the periods indicated follow.

Available for Sale
March 31, 2021	Losses < 12 months				Losses 12 months or >				Total
(in thousands)	Fair Value		Gross Unrealized Losses		Fair Value		Gross Unrealized Losses		Fair Value		Gross Unrealized Losses
U.S. Treasury and government agency securities	$	125,672	$	4,554	$	—	$	—	$	125,672	$	4,554
Municipal obligations		66,344		5,393		—		—		66,344		5,393
Residential mortgage-backed securities		1,669,212		51,668		728		4		1,669,940		51,672
Commercial mortgage-backed securities		1,247,473		64,901		—		—		1,247,473		64,901
Collateralized mortgage obligations		—		—		—		—		—		—
Corporate debt securities		—		—		—		—		—		—
	$	3,108,701	$	126,516	$	728	$	4	$	3,109,429	$	126,520

Available for Sale
December 31, 2020	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

At each reporting period, the Company evaluates its held to maturity municipal obligation portfolio for credit loss using probability of default and loss given default models. The models are run using a long-term average probability of default migration and with a probability weighting of Moody’s economic forecasts. The economic forecasts are largely weighted to a baseline scenario with some weight given to one or more upside and/or downside scenarios. The forecasts are further stressed by running a more severe probability of default migration. The resulting credit loss was negligible for both periods presented 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 for the periods indicated follow.

Held to maturity
March 31, 2021	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		7,690		310		190		1		7,880		311
Residential mortgage-backed securities		33,123		459		—		—		33,123		459
Commercial mortgage-backed securities		37,033		1,041		—		—		37,033		1,041
Collateralized mortgage obligations		—		—		—		—		—		—
	$	77,846	$	1,810	$	190	$	1	$	78,036	$	1,811

Held to maturity
December 31, 2020	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

As of March 31, 2021 and December 31, 2020, the Company had 105 and 28 securities, respectively, with market values below their cost basis. None of the unrealized losses relate primarily to the marketability of the securities or the issuer’s ability 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 deemed to be non-credit related at March 31, 2021 and 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.