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Fair Value Measurements - Summarizes Carrying Value and Estimated Fair Values of Financial Instruments not Carried at Fair Value (Detail) - Surplus Note [Member] - USD ($)
$ in Thousands
Sep. 30, 2018
Dec. 31, 2017
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Carrying Value $ 11,765 $ 12,868
Level 3 [Member]    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]    
Estimated Fair Value $ 10,501 $ 11,630