The following tables provide a summary of the amortized cost and fair values of the available-for-sale securities and held-to-maturity securities at March 31, 2021 and December 31, 2020 (in thousands):

	March 31, 2021
	Amortized Cost		Gross Unrealized Gains ⁽¹⁾		Gross Unrealized Losses ⁽¹⁾			Estimated Fair Value
Available-for-sale securities
U.S. government agency securities	$	3,595	$	50	$	—		$	3,645
State and municipal securities		2,389		39		—			2,428
Mortgage-backed securities:
Agency		997,913		14,592		(8,814	)		1,003,691
Commercial		84,555		2,914		(30	)		87,439
Non-agency		3,648		10		—			3,658
Corporate fixed income securities		695,698		19,702		(3,841	)		711,559
Asset-backed securities		378,201		3,493		(3,986	)		377,708
	$	2,165,999	$	40,800	$	(16,671	)	$	2,190,128
Held-to-maturity securities ⁽²⁾
Asset-backed securities	$	4,761,401	$	10,044	$	(6,421	)	$	4,765,024

	December 31, 2020
	Amortized Cost		Gross Unrealized Gains ⁽¹⁾		Gross Unrealized Losses ⁽¹⁾			Estimated Fair Value
Available-for-sale securities
U.S. government agency securities	$	4,293	$	68	$	—		$	4,361
State and municipal securities		2,395		58		—			2,453
Mortgage-backed securities:
Agency		777,025		16,401		(16	)		793,410
Commercial		91,237		4,376		—			95,613
Non-agency		4,550		28		(9	)		4,569
Corporate fixed income securities		604,662		27,096		—			631,758
Asset-backed securities		700,177		3,522		(5,566	)		698,133
	$	2,184,339	$	51,549	$	(5,591	)	$	2,230,297
Held-to-maturity securities ⁽²⁾
Asset-backed securities	$	4,117,384	$	8,111	$	(17,535	)	$	4,107,960

We are required to evaluate our available-for-sale and held-to-maturity debt securities for any expected losses with recognition of an allowance for credit losses, when applicable. At March 31, 2021, we did not have an allowance for credit losses recorded on our investment portfolio.

Accrued interest receivable for our investment portfolio at March 31, 2021 and December 31, 2020 was $22.5 million and $26.0 million, respectively, and is reported in other assets in the consolidated statements of financial condition. We do not include reserves for interest receivable in the measurement of the allowance for credit losses.

During the three months ended March 31, 2021, we transferred $312.9 million of certain asset-backed securities from available-for-sale to held-to-maturity. Management determined that it has both the positive intent and ability to hold these securities to maturity. The reclassification of these securities was accounted for at fair value. On the date of transfer, the difference between the par value and the fair value of these securities resulted in a premium or discount that, under amortized cost accounting, will be amortized as a yield adjustment to interest income using the interest method. There were no gains or losses recognized as a result of this transfer.

There were no sales of available-for-sale securities during the three months ended March 31, 2021. For the three months ended March 31, 2020, we received proceeds of $288.3 million from the sale of available-for-sale securities, which resulted in a realized loss of $0.7 million.

During the three months ended March 31, 2021 and March 31, 2020, unrealized losses, net of deferred taxes, of $22.7 million and $73.2 million were recorded in accumulated other comprehensive loss in the consolidated statements of financial condition.

The table below summarizes the amortized cost and fair values of our securities by contractual maturity at March 31, 2021 and December 31, 2020 (in thousands). Expected maturities may differ significantly from contractual maturities, as issuers may have the right to call or prepay obligations with or without call or prepayment penalties.

	March 31, 2021				December 31, 2020
	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
Available-for-sale securities
Within one year	$	133,899	$	134,463	$	143,462	$	144,362
After one year through three years		121,840		124,825		134,040		137,625
After three years through five years		242,071		254,774		247,907		266,139
After five years through ten years		348,662		350,871		429,921		435,111
After ten years		1,319,527		1,325,195		1,229,009		1,247,060
	$	2,165,999	$	2,190,128	$	2,184,339	$	2,230,297
Held-to-maturity securities
After three years through five years	$	1,623	$	1,623	$	17,460	$	17,460
After five years through ten years		2,157,703		2,155,117		1,932,439		1,926,425
After ten years		2,602,075		2,608,284		2,167,485		2,164,075
	$	4,761,401	$	4,765,024	$	4,117,384	$	4,107,960

The maturities of our available-for-sale (fair value) and held-to-maturity (amortized cost) securities at March 31, 2021, are as follows (in thousands):

	Within 1 Year		1-5 Years		5-10 Years		After 10 Years		Total
Available-for-sale securities ⁽¹⁾
U.S. government agency securities	$	2,522	$	1,123	$	—	$	—	$	3,645
State and municipal securities		—		—		2,428		—		2,428
Mortgage-backed securities:
Agency		11		272		78,213		925,195		1,003,691
Commercial		10,416		10		—		77,013		87,439
Non-agency		2,970		—		—		688		3,658
Corporate fixed income securities		118,544		378,194		214,821		—		711,559
Asset-backed securities		—		—		55,409		322,299		377,708
	$	134,463	$	379,599	$	350,871	$	1,325,195	$	2,190,128
Held-to-maturity securities
Asset-backed securities	$	—	$	1,623	$	2,157,703	$	2,602,075	$	4,761,401

At March 31, 2021 and December 31, 2020, securities of $534.8 million and $368.4 million, respectively, were pledged at the Federal Home Loan Bank as collateral for borrowings and letters of credit obtained to secure public deposits. At March 31, 2021 and December 31, 2020, securities of $1.0 billion and $1.4 billion, respectively, were pledged with the Federal Reserve discount window.

The following table shows the gross unrealized losses and fair value of the Company’s investment securities with unrealized losses, aggregated by investment category and length of time the individual investment securities have been in continuous unrealized loss positions, at March 31, 2021 (in thousands):

	Less than 12 months					12 months or more					Total
	Gross Unrealized Losses			Estimated Fair Value		Gross Unrealized Losses			Estimated Fair Value		Gross Unrealized Losses			Estimated Fair Value
Available-for-sale securities
Mortgage-backed securities:
Agency	$	(8,814	)	$	489,231	$	—		$	—	$	(8,814	)	$	489,231
Commercial		(30	)		10,416		—			—		(30	)		10,416
Corporate fixed income securities		(3,841	)		143,201		—			—		(3,841	)		143,201
Asset-backed securities		(1,267	)		43,136		(2,719	)		150,571		(3,986	)		193,707
	$	(13,952	)	$	685,984	$	(2,719	)	$	150,571	$	(16,671	)	$	836,555

At March 31, 2021, the amortized cost of 45 securities classified as available for sale exceeded their fair value by $16.7 million, of which $2.7 million related to investment securities that had been in a loss position for 12 months or longer. The total fair value of these investments at March 31, 2021, was $836.6 million, which was 38.2% of our available-for-sale portfolio.

The Company uses Moody credit ratings as the primary credit quality indicator for its held-to-maturity debt securities. Each security is evaluated at least quarterly. The indicators represent the rating for debt securities, as of the date presented, based on the most recent assessment performed. The following table shows the amortized cost of our held-to-maturity securities by credit quality indicator at March 31, 2021 (in thousands):

	AAA		AA		A		C		Total
Held-to-maturity securities
Asset-backed securities	$	1,280,256	$	3,445,799	$	32,855	$	2,491	$	4,761,401