Summary of Investments - Fair Value of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Fixed maturities:
Carrying Value	$ 1,727,601	$ 1,825,438
Fixed maturities
Fixed maturities:
Cost or Amortized Cost	1,655,228	1,720,291
Gross Unrealized Appreciation	80,151	107,003
Gross Unrealized Depreciation	7,778	1,851
Fair Value	1,727,601	1,825,443
Allowance for Credit Losses	0	5
Carrying Value	1,727,601	1,825,438
Short-Term Investments
Fixed maturities:
Cost or Amortized Cost	175	175
Gross Unrealized Appreciation	0
Gross Unrealized Depreciation	0
Fair Value	175
Allowance for Credit Losses	0
Carrying Value	175
Total Available-for-Sale Investments
Fixed maturities:
Cost or Amortized Cost	1,655,403
Gross Unrealized Appreciation	80,151
Gross Unrealized Depreciation	7,778
Fair Value	1,727,776
Allowance for Credit Losses	0
Carrying Value	1,727,776
U.S. Treasury
Fixed maturities:
Cost or Amortized Cost	53,543	149,481
Gross Unrealized Appreciation	283	482
Gross Unrealized Depreciation	448	25
Fair Value	53,378	149,938
Allowance for Credit Losses	0	0
Carrying Value	53,378	149,938
U.S. government agency
Fixed maturities:
Cost or Amortized Cost	55,142	60,502
Gross Unrealized Appreciation	2,646	4,016
Gross Unrealized Depreciation	644	0
Fair Value	57,144	64,518
Allowance for Credit Losses	0	0
Carrying Value	57,144	64,518
States, municipalities and political subdivisions \| General obligations \| Midwest
Fixed maturities:
Cost or Amortized Cost	68,996	77,933
Gross Unrealized Appreciation	2,714	4,047
Gross Unrealized Depreciation	0	0
Fair Value	71,710	81,980
Allowance for Credit Losses	0	0
Carrying Value	71,710	81,980
States, municipalities and political subdivisions \| General obligations \| Northeast
Fixed maturities:
Cost or Amortized Cost	26,562	29,071
Gross Unrealized Appreciation	811	1,379
Gross Unrealized Depreciation	0	0
Fair Value	27,373	30,450
Allowance for Credit Losses	0	0
Carrying Value	27,373	30,450
States, municipalities and political subdivisions \| General obligations \| South
Fixed maturities:
Cost or Amortized Cost	92,091	104,522
Gross Unrealized Appreciation	4,196	5,448
Gross Unrealized Depreciation	0	0
Fair Value	96,287	109,970
Allowance for Credit Losses	0	0
Carrying Value	96,287	109,970
States, municipalities and political subdivisions \| General obligations \| West
Fixed maturities:
Cost or Amortized Cost	95,022	102,590
Gross Unrealized Appreciation	5,549	7,431
Gross Unrealized Depreciation	0	0
Fair Value	100,571	110,021
Allowance for Credit Losses	0	0
Carrying Value	100,571	110,021
States, municipalities and political subdivisions \| Special revenue \| Midwest
Fixed maturities:
Cost or Amortized Cost	118,800	115,956
Gross Unrealized Appreciation	7,473	9,142
Gross Unrealized Depreciation	0	0
Fair Value	126,273	125,098
Allowance for Credit Losses	0	0
Carrying Value	126,273	125,098
States, municipalities and political subdivisions \| Special revenue \| Northeast
Fixed maturities:
Cost or Amortized Cost	55,940	56,317
Gross Unrealized Appreciation	4,072	4,759
Gross Unrealized Depreciation	0	0
Fair Value	60,012	61,076
Allowance for Credit Losses	0	0
Carrying Value	60,012	61,076
States, municipalities and political subdivisions \| Special revenue \| South
Fixed maturities:
Cost or Amortized Cost	201,920	208,739
Gross Unrealized Appreciation	14,723	17,967
Gross Unrealized Depreciation	87	0
Fair Value	216,556	226,706
Allowance for Credit Losses	0	0
Carrying Value	216,556	226,706
States, municipalities and political subdivisions \| Special revenue \| West
Fixed maturities:
Cost or Amortized Cost	127,709	129,417
Gross Unrealized Appreciation	8,119	9,982
Gross Unrealized Depreciation	0	0
Fair Value	135,828	139,399
Allowance for Credit Losses	0	0
Carrying Value	135,828	139,399
Foreign bonds
Fixed maturities:
Cost or Amortized Cost	30,331	27,799
Gross Unrealized Appreciation	1,008	1,805
Gross Unrealized Depreciation	175	2
Fair Value	31,164	29,602
Allowance for Credit Losses	0	0
Carrying Value	31,164	29,602
Public utilities
Fixed maturities:
Cost or Amortized Cost	95,948	76,114
Gross Unrealized Appreciation	5,138	7,388
Gross Unrealized Depreciation	281	0
Fair Value	100,805	83,502
Allowance for Credit Losses	0	0
Carrying Value	100,805	83,502
Corporate bonds \| Energy
Fixed maturities:
Cost or Amortized Cost	28,060	22,441
Gross Unrealized Appreciation	2,263	2,895
Gross Unrealized Depreciation	65	0
Fair Value	30,258	25,336
Allowance for Credit Losses	0	0
Carrying Value	30,258	25,336
Corporate bonds \| Industrials
Fixed maturities:
Cost or Amortized Cost	52,097	39,513
Gross Unrealized Appreciation	2,783	3,744
Gross Unrealized Depreciation	137	0
Fair Value	54,743	43,257
Allowance for Credit Losses	0	0
Carrying Value	54,743	43,257
Corporate bonds \| Consumer goods and services
Fixed maturities:
Cost or Amortized Cost	60,554	46,521
Gross Unrealized Appreciation	2,477	4,046
Gross Unrealized Depreciation	757	0
Fair Value	62,274	50,567
Allowance for Credit Losses	0	0
Carrying Value	62,274	50,567
Corporate bonds \| Health care
Fixed maturities:
Cost or Amortized Cost	27,385	6,678
Gross Unrealized Appreciation	639	898
Gross Unrealized Depreciation	531	0
Fair Value	27,493	7,576
Allowance for Credit Losses	0	0
Carrying Value	27,493	7,576
Corporate bonds \| Technology, media and telecommunications
Fixed maturities:
Cost or Amortized Cost	53,421	37,270
Gross Unrealized Appreciation	3,707	4,381
Gross Unrealized Depreciation	740	15
Fair Value	56,388	41,636
Allowance for Credit Losses	0	0
Carrying Value	56,388	41,636
Corporate bonds \| Financial services
Fixed maturities:
Cost or Amortized Cost	90,400	93,736
Gross Unrealized Appreciation	5,336	7,564
Gross Unrealized Depreciation	308	269
Fair Value	95,428	101,031
Allowance for Credit Losses	0	5
Carrying Value	95,428	101,026
Mortgage-backed securities
Fixed maturities:
Cost or Amortized Cost	22,251	20,305
Gross Unrealized Appreciation	249	326
Gross Unrealized Depreciation	127	54
Fair Value	22,373	20,577
Allowance for Credit Losses	0	0
Carrying Value	22,373	20,577
Collateralized mortgage obligations \| Government national mortgage association
Fixed maturities:
Cost or Amortized Cost	115,576	81,758
Gross Unrealized Appreciation	3,038	4,439
Gross Unrealized Depreciation	1,358	45
Fair Value	117,256	86,152
Allowance for Credit Losses	0	0
Carrying Value	117,256	86,152
Collateralized mortgage obligations \| Federal home loan mortgage corporation
Fixed maturities:
Cost or Amortized Cost	127,461	151,362
Gross Unrealized Appreciation	1,089	2,239
Gross Unrealized Depreciation	1,588	758
Fair Value	126,962	152,843
Allowance for Credit Losses	0	0
Carrying Value	126,962	152,843
Collateralized mortgage obligations \| Federal national mortgage association
Fixed maturities:
Cost or Amortized Cost	55,697	81,952
Gross Unrealized Appreciation	1,252	2,013
Gross Unrealized Depreciation	532	683
Fair Value	56,417	83,282
Allowance for Credit Losses	0	0
Carrying Value	56,417	83,282
Asset-backed securities
Fixed maturities:
Cost or Amortized Cost	322	314
Gross Unrealized Appreciation	586	612
Gross Unrealized Depreciation	0	0
Fair Value	908	926
Allowance for Credit Losses	0	0
Carrying Value	$ 908	$ 926

Debt Securities, Available-for-Sale, Excluding Accrued Interest, Before Allowance For Credit Loss

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- Definition

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
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- Definition

Amount excluding accrued interest, of allowance for credit loss on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
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-URI http://asc.fasb.org/extlink&oid=124258926&loc=SL82898722-210454

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Amortized cost excluding accrued interest, before allowance for credit loss, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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- Definition

Amount excluding accrued interest, of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
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