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Investments - Additional Information (Detail) - USD ($)
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Schedule Of Available For Sale Securities [Line Items]    
Equity securities, Unrealized loss position twelve months or longer $ 28,018,000 $ 30,080,000
Equity securities, Unrealized loss 2,900,000  
Equity Securities [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Equity securities, Unrealized loss position twelve months or longer 11,600,000  
OTTI losses on equity portfolio $ 0