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Investments - Additional Information (Detail) - USD ($)
6 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Schedule Of Available For Sale Securities [Line Items]    
Equity securities, Unrealized loss position twelve months or longer $ 23,387,000 $ 28,018,000
Equity securities, Unrealized loss 2,200,000  
Equity Securities [Member]    
Schedule Of Available For Sale Securities [Line Items]    
Equity securities, Unrealized loss position twelve months or longer 9,142,000  
OTTI losses on equity portfolio $ 0