Short-term investments held at December 31, 2021 include $55.8 million of corporate bonds and $1.6 million of obligations of states and political subdivisions. 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.

The amortized cost, allowance for credit losses, carrying amount, gross unrecognized gains and losses, and the fair value of those investments classified as held-to-maturity at December 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	$	471,688	$	(48	)	$	471,640	$	25,175	$	(263	)	$	496,552
Corporate bonds		56,756		(143	)		56,613		1,344		(114	)		57,843
U.S. agency-based mortgage-backed securities		4,623		—			4,623		377		—			5,000
U.S. Treasury securities and obligations of U.S. government agencies		16,251		—			16,251		132		(36	)		16,347
Asset-backed securities		108		(4	)		104		4		—			108
Totals	$	549,426	$	(195	)	$	549,231	$	27,032	$	(413	)	$	575,850

The amortized cost, gross unrealized gains and losses, fair value, and the allowance for credit losses of those investments classified as available-for-sale at December 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	$	202,008	$	14,538	$	(240	)	$	216,306	$	—
Corporate bonds		93,947		2,751		(272	)		96,426		—
U.S. agency-based mortgage-backed securities		7,944		158		(13	)		8,089		—
U.S. Treasury securities and obligations of U.S. government agencies		20,698		382		(132	)		20,948		—
Totals	$	324,597	$	17,829	$	(657	)	$	341,769	$	—

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

The amortized cost, allowance for credit losses, carrying amount, gross unrealized gains and losses, and the 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

	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 cost, gross unrealized gains and losses on, and fair value of equity securities at December 31, 2020 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, 2021				December 31, 2020
	Carrying Amount		Fair Value		Carrying Amount		Fair Value
	(in thousands)
Maturity:
Within one year	$	70,859	$	71,455	$	54,316	$	54,794
After one year through five years		174,157		180,376		195,706		204,289
After five years through ten years		100,543		104,851		107,347		113,643
After ten years		198,945		214,060		220,345		240,860
U.S. agency-based mortgage-backed securities		4,623		5,000		7,261		7,906
Asset-backed securities		104		108		155		162
Totals	$	549,231	$	575,850	$	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:

	December 31, 2021				December 31, 2020
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
	(in thousands)
Maturity:
Within one year	$	30,925	$	31,258	$	69,177	$	69,938
After one year through five years		97,830		100,716		80,593		85,829
After five years through ten years		58,050		60,137		53,835		57,829
After ten years		129,848		141,569		165,406		181,631
U.S. agency-based mortgage-backed securities		7,944		8,089		18,654		19,052
Totals	$	324,597	$	341,769	$	387,665	$	414,279

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, 2021, there were $23.2 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 2021, 2020 and 2019 is as follows:

	Fixed Maturity Securities Available-for-Sale			Equity Securities		Other			Total
	(in thousands)
Year ended December 31, 2021
Proceeds from sales	$	28,191		$	—	$	12,388		$	40,579
Gross realized investment gains	$	1,751		$	—	$	3		$	1,754
Gross realized investment losses		(21	)		—		—			(21	)
Net realized investment gains		1,730			—		3			1,733
Other, including losses on calls and redemptions		(6	)		—		(32	)		(38	)
Net realized gains (losses) on investments	$	1,724		$	—	$	(29	)	$	1,695
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	)

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

	Year Ended December 31,
	2021			2020			2019
	(in thousands)
Gross investment income:
Fixed maturity securities	$	24,882		$	28,414		$	30,343
Equity securities		1,236			1,006			628
Short-term investments and cash and cash equivalents		304			897			2,478
Total gross investment income		26,422			30,317			33,449
Investment expenses		(987	)		(953	)		(966	)
Net investment income	$	25,435		$	29,364		$	32,483

The following table summarizes the fair value and gross unrealized losses on fixed maturity 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:

	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, 2021
Available-for-Sale
States and political subdivisions	$	23,465	$	240	$	—	$	—	$	23,465	$	240
Corporate bonds		36,443		272		—		—		36,443		272
U.S. agency-based mortgage-backed securities		1,146		13		—		—		1,146		13
U.S. Treasury securities and obligations of U.S. government agencies		6,771		132		—		—		6,771		132
Total available-for-sale securities	$	67,825	$	657	$	—	$	—	$	67,825	$	657
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
Total	$	4,648	$	5	$	—	$	—	$	4,648	$	5

At December 31, 2021, the Company held 42 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.

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, 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)		6		(82	)		—		—		(3	)		(79	)
Balance at December 31, 2021	$	48	$	143		$	—	$	—	$	4		$	195

The Company has established an allowance for credit losses on 372 held-to-maturity securities totaling $0.2 million as of December 31, 2021. The majority of those securities were issued by states and political subdivisions and corporate bonds at 347 and 22, respectively.

The Company has no allowance for credit losses on investments classified as available-for-sale as of December 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 (given a rating), the probability of default 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 (given a rating), the probability of default 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, 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	$	468,698	$	24,719	$	4,623	$	16,251	$	75	$	514,366
Baa/BBB ratings		2,990		32,037		—		—		14		35,041
B ratings		—		—		—		—		19		19
Total	$	471,688	$	56,756	$	4,623	$	16,251	$	108	$	549,426