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Schedule I - Summary of Investments - Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2024 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	$ 9,998,002
Carrying Amount	9,651,297
Short-term securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	509,328
Carrying Amount	509,318
Alternative investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	440,896
Carrying Amount	440,896
Other investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	101,065
Carrying Amount	101,065
Held-to-maturity: | Fixed income securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	25,375
Fair Value	24,735
Carrying Amount	25,375
Held-to-maturity: | All other corporate securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	25,375
Fair Value	24,735
Carrying Amount	25,375
Available-for-sale: | Fixed income securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	8,476,078
Fair Value	8,127,334
Carrying Amount	8,127,334
Available-for-sale: | U.S. government and government agencies
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	139,906
Fair Value	120,155
Carrying Amount	120,155
Available-for-sale: | Foreign government
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	10,656
Fair Value	9,302
Carrying Amount	9,302
Available-for-sale: | Obligations of states and political subdivisions
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	483,609
Fair Value	451,230
Carrying Amount	451,230
Available-for-sale: | Public utilities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	229,834
Fair Value	221,386
Carrying Amount	221,386
Available-for-sale: | All other corporate securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	2,951,212
Fair Value	2,846,794
Carrying Amount	2,846,794
Available-for-sale: | Collateralized loan obligation securities and other asset-backed securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	2,065,611
Fair Value	2,033,149
Carrying Amount	2,033,149
Available-for-sale: | Residential mortgage-backed securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	1,812,744
Fair Value	1,692,358
Carrying Amount	1,692,358
Available-for-sale: | Commercial mortgage-backed securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	782,506
Fair Value	752,960
Carrying Amount	752,960
Equity securities | Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	211,486
Fair Value	213,601
Carrying Amount	213,601
Equity securities | Banks, trusts and insurance companies
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	34,781
Fair Value	33,262
Carrying Amount	33,262
Equity securities | Industrial, miscellaneous and all other
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	174,745
Fair Value	178,505
Carrying Amount	178,505
Equity securities | Preferred stock:
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	1,960
Fair Value	1,834
Carrying Amount	1,834
Commercial mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	233,774
Carrying Amount	$ 233,708

X

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+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Subsection 15
-Publisher SEC

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477271/946-320-S99-7

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Amount of investment held by insurance company, excluding investment in related party.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15(Column D))
-Publisher FASB
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Reference 1: http://www.xbrl.org/2003/role/disclosureRef
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-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 7
-Subparagraph (SX 210.12-15(Column B))
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+ References

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