Summary of Investments - Fair Value of Investments (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Fixed maturities:
Amortized Cost	$ 1,794,325	$ 1,662,680
Gross Unrealized Appreciation	525	2,784
Gross Unrealized Depreciation	156,212	114,125
Fair Value	1,638,638	1,551,339
Allowance for Credit Losses	126	3
Carrying Value	1,638,512	1,551,336
U.S. Treasury
Fixed maturities:
Amortized Cost	68,133	15,684
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	1,260	1,009
Fair Value	66,873	14,675
Allowance for Credit Losses	0	0
Carrying Value	66,873	14,675
U.S. government agency
Fixed maturities:
Amortized Cost	104,695	94,092
Gross Unrealized Appreciation	16	35
Gross Unrealized Depreciation	12,813	9,721
Fair Value	91,898	84,406
Allowance for Credit Losses	0	0
Carrying Value	91,898	84,406
General obligations \| Midwest
Fixed maturities:
Amortized Cost	58,724	61,191
Gross Unrealized Appreciation	0	185
Gross Unrealized Depreciation	1,843	263
Fair Value	56,881	61,113
Allowance for Credit Losses	0	0
Carrying Value	56,881	61,113
General obligations \| Northeast
Fixed maturities:
Amortized Cost	11,440	15,518
Gross Unrealized Appreciation	0	18
Gross Unrealized Depreciation	441	73
Fair Value	10,999	15,463
Allowance for Credit Losses	0	0
Carrying Value	10,999	15,463
General obligations \| South
Fixed maturities:
Amortized Cost	55,335	64,851
Gross Unrealized Appreciation	0	57
Gross Unrealized Depreciation	2,201	927
Fair Value	53,134	63,981
Allowance for Credit Losses	0	0
Carrying Value	53,134	63,981
General obligations \| West
Fixed maturities:
Amortized Cost	78,044	87,094
Gross Unrealized Appreciation	0	163
Gross Unrealized Depreciation	2,718	712
Fair Value	75,326	86,545
Allowance for Credit Losses	0	0
Carrying Value	75,326	86,545
Special revenue \| Midwest
Fixed maturities:
Amortized Cost	101,204	103,107
Gross Unrealized Appreciation	1	224
Gross Unrealized Depreciation	4,327	1,065
Fair Value	96,878	102,266
Allowance for Credit Losses	0	0
Carrying Value	96,878	102,266
Special revenue \| Northeast
Fixed maturities:
Amortized Cost	52,836	55,292
Gross Unrealized Appreciation	0	76
Gross Unrealized Depreciation	2,284	1,148
Fair Value	50,552	54,220
Allowance for Credit Losses	0	0
Carrying Value	50,552	54,220
Special revenue \| South
Fixed maturities:
Amortized Cost	168,395	184,108
Gross Unrealized Appreciation	0	278
Gross Unrealized Depreciation	8,895	3,529
Fair Value	159,500	180,857
Allowance for Credit Losses	0	0
Carrying Value	159,500	180,857
Special revenue \| West
Fixed maturities:
Amortized Cost	105,521	113,594
Gross Unrealized Appreciation	0	275
Gross Unrealized Depreciation	5,005	1,657
Fair Value	100,516	112,212
Allowance for Credit Losses	0	0
Carrying Value	100,516	112,212
Foreign bonds
Fixed maturities:
Amortized Cost	21,251	36,129
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	2,915	4,480
Fair Value	18,336	31,649
Allowance for Credit Losses	0	0
Carrying Value	18,336	31,649
Public utilities
Fixed maturities:
Amortized Cost	143,162	138,752
Gross Unrealized Appreciation	14	65
Gross Unrealized Depreciation	15,759	13,406
Fair Value	127,417	125,411
Allowance for Credit Losses	0	0
Carrying Value	127,417	125,411
Corporate bonds \| Energy
Fixed maturities:
Amortized Cost	45,368	36,507
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	3,768	3,298
Fair Value	41,600	33,209
Allowance for Credit Losses	0	0
Carrying Value	41,600	33,209
Corporate bonds \| Industrials
Fixed maturities:
Amortized Cost	73,635	58,334
Gross Unrealized Appreciation	38	62
Gross Unrealized Depreciation	7,705	5,554
Fair Value	65,968	52,842
Allowance for Credit Losses	0	0
Carrying Value	65,968	52,842
Corporate bonds \| Consumer goods and services
Fixed maturities:
Amortized Cost	100,300	100,539
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	11,794	10,598
Fair Value	88,506	89,941
Allowance for Credit Losses	0	0
Carrying Value	88,506	89,941
Corporate bonds \| Health care
Fixed maturities:
Amortized Cost	37,903	32,987
Gross Unrealized Appreciation	0	24
Gross Unrealized Depreciation	6,366	5,419
Fair Value	31,537	27,592
Allowance for Credit Losses	0	0
Carrying Value	31,537	27,592
Corporate bonds \| Technology, media and telecommunications
Fixed maturities:
Amortized Cost	90,285	67,193
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	10,037	7,253
Fair Value	80,248	59,940
Allowance for Credit Losses	0	0
Carrying Value	80,248	59,940
Corporate bonds \| Financial services
Fixed maturities:
Amortized Cost	145,477	132,849
Gross Unrealized Appreciation	0	851
Gross Unrealized Depreciation	12,315	9,408
Fair Value	133,162	124,292
Allowance for Credit Losses	126	3
Carrying Value	133,036	124,289
Mortgage-backed securities
Fixed maturities:
Amortized Cost	24,470	20,450
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	3,463	2,750
Fair Value	21,007	17,700
Allowance for Credit Losses	0	0
Carrying Value	21,007	17,700
Collateralized mortgage obligations \| Government national mortgage association
Fixed maturities:
Amortized Cost	163,893	97,839
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	17,223	13,291
Fair Value	146,670	84,548
Allowance for Credit Losses	0	0
Carrying Value	146,670	84,548
Collateralized mortgage obligations \| Federal home loan mortgage corporation
Fixed maturities:
Amortized Cost	88,565	92,366
Gross Unrealized Appreciation	0	0
Gross Unrealized Depreciation	16,716	13,528
Fair Value	71,849	78,838
Allowance for Credit Losses	0	0
Carrying Value	71,849	78,838
Collateralized mortgage obligations \| Federal national mortgage association
Fixed maturities:
Amortized Cost	52,114	50,272
Gross Unrealized Appreciation	0	5
Gross Unrealized Depreciation	6,227	4,891
Fair Value	45,887	45,386
Allowance for Credit Losses	0	0
Carrying Value	45,887	45,386
Asset-backed securities
Fixed maturities:
Amortized Cost	3,575	3,932
Gross Unrealized Appreciation	456	466
Gross Unrealized Depreciation	137	145
Fair Value	3,894	4,253
Allowance for Credit Losses	0	0
Carrying Value	$ 3,894	$ 4,253

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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

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- Definition

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://www.xbrl.org/2003/role/disclosureRef
-Topic 320
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481800/320-10-50-2

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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
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479130/326-30-45-1

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- Definition

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
-Topic 326
-SubTopic 30
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479130/326-30-45-1

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