Investments - Additional Information (Detail)	9 Months Ended
Sep. 30, 2020 USD ($) Entity	Dec. 31, 2019 USD ($)
Debt Securities, Available-for-sale [Line Items]
Accrued interest receivable	$ 6,100,000	$ 7,000,000.0
Investments in a single issuer as a percentage of shareholders' equity	2.00%	3.00%
Fixed maturity securities with market value	$ 0	$ 0
Investments in insurance enhanced bonds	$ 26,000,000.0
Insurance enhanced bonds as a percentage of total cash and invested assets	1.80%
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	$ 26,000,000.0
Variable Interest Entity, Not Primary Beneficiary
Debt Securities, Available-for-sale [Line Items]
Number of VIE's \| Entity	3
Ownership interest exceeds respective investments	3.00%
One of the Company's variable interest VIE's, invests in distressed securities and assets
Debt Securities, Available-for-sale [Line Items]
Significant variable interest in carrying value of the non-consolidated VIE	$ 11,300,000	13,500,000
Variable interest entities, maximum exposure to loss	25,500,000	27,700,000
Second VIE that invests in distressed securities and assets
Debt Securities, Available-for-sale [Line Items]
Significant variable interest in carrying value of the non-consolidated VIE	17,000,000.0	24,000,000.0
Variable interest entities, maximum exposure to loss	34,000,000.0	41,000,000.0
Third VIE that invests in REIT qualifying assets
Debt Securities, Available-for-sale [Line Items]
Significant variable interest in carrying value of the non-consolidated VIE	9,500,000	9,800,000
Variable interest entities, maximum exposure to loss	9,500,000	$ 10,300,000
Municipal Bond Insurance Association
Debt Securities, Available-for-sale [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	3,200,000
Assured Guaranty Corporation
Debt Securities, Available-for-sale [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	$ 10,000,000.0
Fannie Mae Mortgage Pools
Debt Securities, Available-for-sale [Line Items]
Investment in mortgage pools as percentage of shareholders' equity	4.60%	4.20%
Municipal Bonds
Debt Securities, Available-for-sale [Line Items]
Investments in insurance enhanced bonds	$ 15,300,000
Federal Home Loan Mortgage Corporation
Debt Securities, Available-for-sale [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	10,600,000
Ambac Financial Group
Debt Securities, Available-for-sale [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	2,200,000
Federal Deposit Insurance Corporation \| Maximum
Debt Securities, Available-for-sale [Line Items]
Investments in collateralized mortgage obligations, commercial mortgage-backed securities and taxable municipal bonds	100,000
Commercial Mortgage-Backed Securities
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	$ 804,000
Weighted average credit enhancement	32.40%
Investments in insurance enhanced bonds	$ 10,600,000
Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	1,135,000
U.S. Treasury and Agency Obligations
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	215,000
Obligations of States and Political Subdivisions
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	188,000
Mortgage Backed Securities
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	1,120,000
Asset-backed Securities
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	$ 1,001,000.000
Weighted average credit enhancement	33.30%
Foreign Corporate Bonds
Debt Securities, Available-for-sale [Line Items]
Gross unrealized losses	$ 204,000
Collateralized Mortgage Obligations \| Maximum
Debt Securities, Available-for-sale [Line Items]
Investments in insurance enhanced bonds	$ 100,000

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