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Fair Value Measurements - Additional Information (Detail) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
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Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items] |  |
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities | $ 93 | $ 138
Other investments | 428,556 | 391,217
Derivative liabilities | $ 86,923 | $ 76,999
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs | 2.60% | 2.70%
Fair Value Measured at Net Asset Value Per Share [Member] |  |
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items] |  |
Other investments | $ 284,788 | $ 279,941
Fixed Income Securities [Member] |  |
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items] |  |
Percentage of investment portfolio valued using dealer quotes | 8.00% | 8.00%
Minimum [Member] | Fixed Income Securities [Member] |  |
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items] |  |
Percentage of investment portfolio valued using internal valuation models | 2.00% | 2.00%
Maximum [Member] | Fixed Income Securities [Member] |  |
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items] |  |
Percentage of investment portfolio valued using external pricing services | 90.00% | 90.00%
Variable Interest Entity [Member] |  |
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items] |  |
Weighted average discounted rate of estimated future premium payments to be paid by the VIEs | 3.00% | 3.10%
Variable Interest Entity, Primary Beneficiary [Member] |  |
Fair Value Of Assets And Liabilities Measured On Non Recurring Basis [Line Items] |  |
Derivative liabilities | $ 1,781,903 | $ 1,712,062

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