Investments - Additional Information (Detail)	9 Months Ended
Sep. 30, 2021 USD ($) Entity	Dec. 31, 2020 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Investments in a single issuer as a percentage of shareholders' equity	2.00%	1.90%
Accrued interest receivable	$ 5,500,000	$ 5,700,000
Fixed maturity securities with market value	0	0
Investments in insurance enhanced bonds	$ 28,300,000
Insurance enhanced bonds as a percentage of total cash and invested assets	1.90%
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	$ 28,300,000
Variable Interest Entity, Not Primary Beneficiary
Schedule Of Available For Sale Securities [Line Items]
Number of VIE's \| Entity	4
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	$ 9,300,000	10,800,000
Variable interest entities, maximum exposure to loss	23,500,000	25,000,000.0
Second VIE that 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	2,900,000	15,700,000
Variable interest entities, maximum exposure to loss	19,900,000	32,700,000
Third VIE that invests in Real Estate Investment Trust (REIT) qualifying assets
Schedule Of Available For Sale Securities [Line Items]
Significant variable interest in carrying value of the non-consolidated VIE	11,600,000	10,500,000
Variable interest entities, maximum exposure to loss	11,600,000	10,500,000
Fourth VIE that invests in Broad Portfolio of Non-Investment Grade Loans
Schedule Of Available For Sale Securities [Line Items]
Significant variable interest in carrying value of the non-consolidated VIE	106,200,000	60,000,000.0
Variable interest entities, maximum exposure to loss	106,200,000	$ 60,000,000.0
Municipal Bond Insurance Association
Schedule Of Available For Sale Securities [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	2,700,000
Assured Guaranty Corporation
Schedule Of Available For Sale Securities [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	8,200,000
U.S. Treasuries
Schedule Of Available For Sale Securities [Line Items]
Gross unrealized losses	1,632,000
Agency Obligations
Schedule Of Available For Sale Securities [Line Items]
Gross unrealized losses	151,000
Obligations of States and Political Subdivisions
Schedule Of Available For Sale Securities [Line Items]
Gross unrealized losses	163,000
Mortgage Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Gross unrealized losses	1,417,000
Asset-backed Securities
Schedule Of Available For Sale Securities [Line Items]
Gross unrealized losses	$ 436,000
Weighted average credit enhancement	32.40%
Commercial Mortgage-Backed Securities
Schedule Of Available For Sale Securities [Line Items]
Gross unrealized losses	$ 266,000
Weighted average credit enhancement	39.40%
Investments in insurance enhanced bonds	$ 6,800,000
Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Gross unrealized losses	1,249,000
Foreign Corporate Bonds
Schedule Of Available For Sale Securities [Line Items]
Gross unrealized losses	342,000
Collateralized Mortgage Obligations
Schedule Of Available For Sale Securities [Line Items]
Investments in insurance enhanced bonds	$ 7,800,000
Mortgage Pools
Schedule Of Available For Sale Securities [Line Items]
Investment in mortgage pools as percentage of shareholders' equity	2.10%	3.90%
Municipal Bonds
Schedule Of Available For Sale Securities [Line Items]
Investments in insurance enhanced bonds	$ 13,700,000
Federal Home Loan Mortgage Corporation
Schedule Of Available For Sale Securities [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	14,600,000
Federal National Mortgage Association
Schedule Of Available For Sale Securities [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	300,000
Ambac Financial Group
Schedule Of Available For Sale Securities [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	1,900,000
School Bond Guaranty Program
Schedule Of Available For Sale Securities [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	100,000
Higher Education State Aid Intercept Program
Schedule Of Available For Sale Securities [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	$ 500,000

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