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Fair Value Measurements - Summarizes Carrying Value and Estimated Fair Values of Financial Instruments not Carried at Fair Value (Detail) - USD ($)
$ in Thousands
Dec. 31, 2018
Dec. 31, 2017
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]    
Carrying value of surplus note $ 11,397 $ 12,868
Surplus Note | Level 3    
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]    
Estimated Fair Value $ 10,125 $ 11,630