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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, 2020
Dec. 31, 2019
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]    
Surplus note $ 8,456 $ 9,926
Surplus Note [Member] | Level 3 [Member]    
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]    
Estimated fair value of surplus note $ 8,291 $ 9,365