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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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