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Investments - Additional Information (Detail) - USD ($) | 9 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2023 | Dec. 31, 2021 | Dec. 31, 2022
Schedule Of Available For Sale Securities [Line Items] |  |  |
Investments in a single issuer as a percentage of shareholders' equity | 2.10% |  | 2.40%
Weighted average life of fixed maturities securities | 5 years |  |
Maturity period of securities that reinvested to fixed income investments | 2 years |  |
Percentage of book yield due to reinvested to fixed income investments | 4.00% | 2.20% |
Fixed maturity securities with market value | $ 0 |  | $ 0
Investments in insurance enhanced bonds | $ 4,900,000 |  |
Insurance enhanced bonds as a percentage of total cash and invested assets | 0.40% |  |
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds | $ 4,900,000 |  |
Variable Interest Entity, Not Primary Beneficiary |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Ownership interest exceeds respective investments | 3.00% |  |
One of the Company's variable interest VIE's, invests in distressed securities and assets |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Significant variable interest in carrying value of the non-consolidated VIE | $ 4,100,000 |  | 4,800,000
Variable interest entities, maximum exposure to loss | 18,300,000 |  | 19,000,000
Variable interest entities, maximum exposure to loss | 4,100,000 |  | 4,800,000
Second VIE that invests in REIT qualifying assets |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Significant variable interest in carrying value of the non-consolidated VIE | 9,100,000 |  | 9,800,000
Variable interest entities, maximum exposure to loss | 9,100,000 |  | 9,800,000
Assured Guaranty Corporation |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds | 3,700,000 |  |
Ambac Financial Group |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds | 1,200,000 |  |
Other Assets |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Accrued interest receivable related to fixed maturities | 6,500,000 |  | $ 8,400,000
U.S. Treasuries |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Gross unrealized losses | 6,671,000 |  |
Obligations of States and Political Subdivisions |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Gross unrealized losses | 1,760,000 |  |
Mortgage Backed Securities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Gross unrealized losses | 7,487,000 |  |
Asset-backed Securities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Gross unrealized losses | $ 7,124,000 |  |
Weighted average credit enhancement | 35.90% |  |
Commercial Mortgage-Backed Securities |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Gross unrealized losses | $ 6,628,000 |  |
Weighted average credit enhancement | 47.10% |  |
Corporate Bonds |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Gross unrealized losses | $ 11,351,000 |  |
Foreign Corporate Bonds |  |  |
Schedule Of Available For Sale Securities [Line Items] |  |  |
Gross unrealized losses | $ 6,236,000 |  |

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