The gross unrecognized gains and losses, amortized cost, allowance for credit losses, carrying amount, and fair value of those investments classified as held-to-maturity at March 31, 2021 are summarized as follows:

	Amortized Cost		Allowance for Credit Losses			Carrying Amount		Gross Unrecognized Gains		Gross Unrecognized Losses			Fair Value
	(in thousands)
States and political subdivisions	$	486,376	$	(44	)	$	486,332	$	26,179	$	(774	)	$	511,737
Corporate bonds		68,518		(116	)		68,402		2,440		—			70,842
U.S. agency-based mortgage-backed securities		6,357		—			6,357		568		—			6,925
U.S. Treasury securities and obligations of U.S. government agencies		13,877		—			13,877		195		(2	)		14,070
Asset-backed securities		146		(6	)		140		6		—			146
Totals	$	575,274	$	(166	)	$	575,108	$	29,388	$	(776	)	$	603,720

The gross unrealized gains and losses, and the amortized cost, allowance for credit losses, and fair value of those investments classified as available-for-sale at March 31, 2021 are summarized as follows:

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value		Allowance for Credit Losses
	(in thousands)
States and political subdivisions	$	248,904	$	15,995	$	(402	)	$	264,497	$	—
Corporate bonds		92,469		4,084		(180	)		96,373		—
U.S. agency-based mortgage-backed securities		14,696		340		(8	)		15,028		—
U.S. Treasury securities and obligations of U.S. government agencies		35,618		650		(262	)		36,006		—
Totals	$	391,687	$	21,069	$	(852	)	$	411,904	$	—

The gross unrealized gains and losses, and the cost of equity securities at March 31, 2021 are summarized as follows:

	Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
	(in thousands)
Equity securities:
Domestic common stock	$	37,682	$	13,161	$	—	$	50,843
Total equity securities	$	37,682	$	13,161	$	—	$	50,843

The gross unrecognized gains and losses, amortized cost, allowance for credit losses, carrying amount, and fair value of those investments classified as held-to-maturity at December 31, 2020 are summarized as follows:

	Amortized Cost		Allowance for Credit Losses			Carrying Amount		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
	(in thousands)
States and political subdivisions	$	494,374	$	(42	)	$	494,332	$	32,489	$	—	$	526,821
Corporate bonds		69,981		(225	)		69,756		3,144		—		72,900
U.S. agency-based mortgage-backed securities		7,261		—			7,261		645		—		7,906
U.S. Treasury securities and obligations of U.S. government agencies		13,626		—			13,626		239		—		13,865
Asset-backed securities		162		(7	)		155		7		—		162
Totals	$	585,404	$	(274	)	$	585,130	$	36,524	$	—	$	621,654

The gross unrealized gains and losses, and the amortized cost and fair value of those investments classified as available-for-sale at December 31, 2020 are summarized as follows:

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
States and political subdivisions	$	256,492	$	20,050	$	—		$	276,542
Corporate bonds		83,646		5,256		(3	)		88,899
U.S. agency-based mortgage-backed securities		18,654		400		(2	)		19,052
U.S. Treasury securities and obligations of U.S. government agencies		28,873		913		—			29,786
Totals	$	387,665	$	26,619	$	(5	)	$	414,279

The gross unrealized gains and losses, and the cost of equity securities at December 31, 2020 are summarized as follows:

	Cost		Gross Unrealized Gains		Gross Unrealized Losses		Fair Value
	(in thousands)
Equity securities:
Domestic common stock	$	35,787	$	7,650	$	—	$	43,437
Total equity securities	$	35,787	$	7,650	$	—	$	43,437

A summary of the carrying amounts and fair value of investments in fixed maturity securities, classified as held-to-maturity, by contractual maturity, is as follows:

	March 31, 2021				December 31, 2020
	Carrying Amount		Fair Value		Carrying Amount		Fair Value
	(in thousands)
Maturity:
Within one year	$	55,462	$	55,896	$	54,316	$	54,794
After one year through five years		199,837		207,924		195,706		204,289
After five years through ten years		101,316		104,986		107,347		113,643
After ten years		211,996		227,843		220,345		240,860
U.S. agency-based mortgage-backed securities		6,357		6,925		7,261		7,906
Asset-backed securities		140		146		155		162
Totals	$	575,108	$	603,720	$	585,130	$	621,654

A summary of the amortized cost and fair value of investments in fixed maturity securities, classified as available-for-sale, by contractual maturity, is as follows:

	March 31, 2021				December 31, 2020
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
	(in thousands)
Maturity:
Within one year	$	62,627	$	63,083	$	69,177	$	69,938
After one year through five years		89,766		94,312		80,593		85,829
After five years through ten years		67,212		69,638		53,835		57,829
After ten years		157,386		169,843		165,406		181,631
U.S. agency-based mortgage-backed securities		14,696		15,028		18,654		19,052
Totals	$	391,687	$	411,904	$	387,665	$	414,279

The following table summarizes the fair value and gross unrealized losses on securities classified as available-for-sale, aggregated by major investment category and length of time that the individual securities have been in a continuous unrealized loss position as of March 31, 2021:

	Less Than 12 Months				12 Months or Greater				Total
	Fair Value of Investments with Unrealized Losses		Gross Unrealized Losses		Fair Value of Investments with Unrealized Losses		Gross Unrealized Losses		Fair Value of Investments with Unrealized Losses		Gross Unrealized Losses
	(in thousands)
March 31, 2021
Available-for-Sale
States and political subdivisions	$	21,199	$	402	$	—	$	—	$	21,199	$	402
Corporate bonds		8,678		180		—		—		8,678		180
U.S. agency-based mortgage-backed securities		1,595		8		—		—		1,595		8
U.S. Treasury securities and obligations of U.S. government agencies		11,492		262		—		—		11,492		262
Total available-for-sale securities	$	42,964	$	852	$	—	$	—	$	42,964	$	852

At March 31, 2021, we held 29 individual fixed maturity securities classified as available-for-sale that were in an unrealized loss position, of which none were in a continuous unrealized loss position for longer than 12 months.

	Less Than 12 Months				12 Months or Greater				Total
	Fair Value of Investments with Unrealized Losses		Gross Unrealized Losses		Fair Value of Investments with Unrealized Losses		Gross Unrealized Losses		Fair Value of Investments with Unrealized Losses		Gross Unrealized Losses
	(in thousands)
December 31, 2020
Available-for-Sale
Corporate bonds	$	2,515	$	3	$	—	$	—	$	2,515	$	3
U.S. agency-based mortgage-backed securities		2,133		2		—		—		2,133		2
Total available-for-sale securities	$	4,648	$	5	$	—	$	—	$	4,648	$	5

The following table illustrates the changes in the allowance for credit losses by major security type of the investments classified as held-to-maturity for the quarter ended March 31, 2021.

	States and Political Subdivisions		Corporate Bonds			U.S. Agency- Based Mortgage- Backed Securities		U.S. Treasury Securities and Obligations of U.S. Government Agencies		Asset- Backed Securities			Totals
	(in thousands)
Balance at December 31, 2020	$	42	$	225		$	—	$	—	$	7		$	274
Provision for credit loss expense (benefit)		2		(109	)		—		—		(1	)		(108	)
Balance at March 31, 2021	$	44	$	116		$	—	$	—	$	6		$	166

The Company has established an allowance for credit losses on 410 held-to-maturity securities totaling $0.2 million. The majority of those securities were issued by states and political subdivisions (380 securities) and corporate bonds (27 securities).

The Company has no allowance for credit losses on investments classified as available-for-sale for the period ended March 31, 2021.

The credit rating used for held-to-maturity fixed income securities is the rating for each security as published by Moody’s, S&P, and Fitch to determine the probability of default. If there are two ratings, the lower rating is used. If there are three ratings, the median rating is used. If there is one rating, that rating is used. For corporate fixed income securities, the probability of default (given a rating) comes from Moody’s annual study of corporate bond defaults published each February. The maximum maturity using the default rate is 20 years (any maturity greater than 20 years will use the 20-year rate). For municipal fixed income securities the probability of default (given a rating) comes from Moody’s annual study of municipal bond defaults published each July/August.

The calculation of the credit loss allowance takes the amortized cost of the fixed income security and assumes default and recovery based on the average recovery rates from the Moody’s default studies. The amortized cost of the security, minus the amount recovered, is the estimated full amount the Company could lose in a default scenario. Then this amount is multiplied by the probability of default to determine the allowance for credit loss. The lower the security is rated, the higher likelihood of default, and therefore a higher allowance for credit loss. The longer to the maturity date of a security, the higher the default risk.

The table below presents the amortized cost of held-to-maturity securities aggregated by credit quality indicator as of March 31, 2021.

	States and Political Subdivisions		Corporate Bonds		U.S. Agency-Based Mortgage-Backed Securities		U.S. Treasury Securities and Obligations of U.S. Government Agencies		Asset-Backed Securities		Totals
	Amortized cost
	(in thousands)
AAA/AA/A ratings	$	481,662	$	29,708	$	6,357	$	13,877	$	100	$	531,704
Baa/BBB ratings		4,714		38,810		—		—		17		43,541
B ratings		—		—		—		—		29		29
Total	$	486,376	$	68,518	$	6,357	$	13,877	$	146	$	575,274

Net realized gains in the quarter ended March 31, 2021 were $0.3 million resulting from the sale of fixed maturity securities classified as available-for-sale. Net realized gains in the quarter ended March 31, 2020 were $1.0 million resulting from the sale of fixed maturity securities classified as available-for-sale.

During the first quarter of 2021, we recognized through income $5.5 million of net unrealized gains on equity securities held as of March 31, 2021. During the first quarter of 2020, we recognized through income $8.8 million of net unrealized losses on equity securities held as of March 31, 2020.

Investment income is recognized as it is earned. The discount or premium on fixed maturity securities is amortized using the “constant yield” method. Anticipated prepayments, where applicable, are considered when determining the amortization of premiums or discounts. Realized investment gains and losses are determined using the specific identification method.