Short-term investments held at December 31, 2020 include $30.7 million of U.S. Treasury securities and obligations of U.S. government agencies, $9.1 million of obligations of states and political subdivisions and $6.1 million of corporate bonds. Short-term investments held at December 31, 2019 include $54.6 million of U.S. Treasury securities and obligations of U.S. government agencies and $1.9 million of obligations of states and political subdivisions.

The gross unrecognized gains and losses on, and the 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 Unrecognized Gains		Gross Unrecognized 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 on, 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		Allowance for Credit Losses
	(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 on, and the cost of equity securities at December 31, 2020 are summarized as follows:

The gross unrealized gains and losses on, and the amortized cost and fair value of, those investments classified as held-to-maturity at December 31, 2019 are summarized as follows:

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
States and political subdivisions	$	466,270	$	19,570	$	(193	)	$	485,647
Corporate bonds		109,241		1,684		—			110,925
U.S. agency-based mortgage-backed securities		10,967		544		—			11,511
U.S. Treasury securities and obligations of U.S. government agencies		12,723		330		(12	)		13,041
Asset-backed securities		220		—		(1	)		219
Totals	$	599,421	$	22,128	$	(206	)	$	621,343

The gross unrealized gains and losses on, and the amortized cost and fair value of, those investments classified as available-for-sale at December 31, 2019 are summarized as follows:

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	(in thousands)
States and political subdivisions	$	225,895	$	11,906	$	(26	)	$	237,775
Corporate bonds		130,453		3,326		(1	)		133,778
U.S. agency-based mortgage-backed securities		29,499		64		(96	)		29,467
U.S. Treasury securities and obligations of U.S. government agencies		39,851		317		(42	)		40,126
Totals	$	425,698	$	15,613	$	(165	)	$	441,146

The gross unrealized gains and losses on, and the cost of equity securities at December 31, 2019 are summarized as follows:

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:

	December 31, 2020				December 31, 2019
	Carrying Amount		Fair Value		Carrying Amount		Fair Value
	(in thousands)
Maturity:
Within one year	$	54,316	$	54,794	$	49,967	$	50,348
After one year through five years		195,706		204,289		198,025		202,109
After five years through ten years		107,347		113,643		110,460		113,877
After ten years		220,345		240,860		229,782		243,279
U.S. agency-based mortgage-backed securities		7,261		7,906		10,967		11,511
Asset-backed securities		155		162		220		219
Totals	$	585,130	$	621,654	$	599,421	$	621,343

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:

	December 31, 2020				December 31, 2019
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
	(in thousands)
Maturity:
Within one year	$	69,177	$	69,938	$	27,160	$	27,194
After one year through five years		80,593		85,829		144,142		146,469
After five years through ten years		53,835		57,829		47,175		49,419
After ten years		165,406		181,631		177,722		188,597
U.S. agency-based mortgage-backed securities		18,654		19,052		29,499		29,467
Totals	$	387,665	$	414,279	$	425,698	$	441,146

Actual maturities may differ from contractual maturities because certain borrowers have the right to call or prepay certain obligations with or without call or prepayment penalties.

At December 31, 2020, there were $19.9 million of held-to-maturity investments on deposit with regulatory agencies of states in which the Company does business.

A summary of the Company’s realized gains and losses on sales, calls or redemptions of investments for 2020, 2019 and 2018 is as follows:

	Fixed Maturity Securities Available-for-Sale			Equity Securities		Other			Total
	(in thousands)
Year ended December 31, 2020
Proceeds from sales	$	25,436		$	—	$	46,959		$	72,395
Gross realized investment gains	$	711		$	—	$	306		$	1,017
Gross realized investment losses		(14	)		—		(4	)		(18	)
Net realized investment gains		697			—		302			999
Other, including gains (losses) on calls and redemptions		(43	)		—		176			133
Net realized gains on investments	$	654		$	—	$	478		$	1,132
Year ended December 31, 2019
Proceeds from sales	$	33,113		$	—	$	21,977		$	55,090
Gross realized investment gains	$	140		$	—	$	36		$	176
Gross realized investment losses		(68	)		—		—			(68	)
Net realized investment gains		72			—		36			108
Other, including gains (losses) on calls and redemptions		(196	)		—		8			(188	)
Net realized gains (losses) on investments	$	(124	)	$	—	$	44		$	(80	)
Year ended December 31, 2018
Proceeds from sales	$	15,025		$	3	$	—		$	15,028
Gross realized investment gains	$	238		$	1	$	—		$	239
Gross realized investment losses		(1,354	)		—		—			(1,354	)
Net realized investment gains (losses)		(1,116	)		1		—			(1,115	)
Other, including losses on calls and redemptions		(144	)		—		(277	)		(421	)
Net realized gains (losses) on investments	$	(1,260	)	$	1	$	(277	)	$	(1,536	)

Major categories of the Company’s net investment income are summarized as follows:

	Year Ended December 31,
	2020			2019			2018
	(in thousands)
Gross investment income:
Fixed maturity securities	$	28,414		$	30,343		$	28,762
Equity securities		1,006			628			448
Short-term investments and cash and cash equivalents		897			2,478			2,208
Total gross investment income		30,317			33,449			31,418
Investment expenses		(953	)		(966	)		(966	)
Net investment income	$	29,364		$	32,483		$	30,452

The following table summarizes the fair value and gross unrealized losses on fixed maturity securities, aggregated by major investment category and length of time that the individual securities have been in a continuous unrealized loss position:

	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
December 31, 2019
Held-to-Maturity
States and political subdivisions	$	21,074	$	193	$	—	$	—	$	21,074	$	193
U.S. Treasury securities and obligations of U.S. government agencies		—		—		3,243		12		3,243		12
Asset-backed securities		—		—		68		1		68		1
Total held-to-maturity securities		21,074		193		3,311		13		24,385		206
Available-for-Sale
States and political subdivisions	$	4,140	$	26	$	—	$	—	$	4,140	$	26
Corporate bonds		6,426		1		—		—		6,426		1
U.S. agency-based mortgage-backed securities		13,007		95		1,152		1		14,159		96
U.S. Treasury securities and obligations of U.S. government agencies		—		—		17,068		42		17,068		42
Total available-for-sale securities		23,573		122		18,220		43		41,793		165
Total	$	44,647	$	315	$	21,531	$	56	$	66,178	$	371

At December 31, 2020, the Company held 2 individual fixed maturity securities that were in an unrealized loss position, of which none were in a continuous unrealized loss position for longer than 12 months.

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 year ended December 31, 2020.

	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 January 1, 2020	$	45		$	245		$	—	$	—	$	11		$	301
Provision for credit loss benefit		(3	)		(20	)		—		—		(4	)		(27	)
Balance at December 31, 2020	$	42		$	225		$	—	$	—	$	7		$	274

The Company has established an allowance for credit losses on 424 held-to-maturity securities totaling $0.3 million as of December 31, 2020. The majority of those securities were states and political subdivisions and corporate bonds at 393 and 28, respectively.

The Company has no allowance for credit losses on investments classified as available-for-sale as of December 31, 2020. During 2019, the Company had no impairment losses recognized for other-than-temporary declines in the fair value of our investments.

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 December 31, 2020.

	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	$	490,311	$	29,890	$	7,261	$	13,626	$	111	$	541,199
Baa/BBB ratings		4,063		40,091		—		—		18		44,172
B ratings		—		—		—		—		33		33
Total	$	494,374	$	69,981	$	7,261	$	13,626	$	162	$	585,404