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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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