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Investments OTTI Credit Losses Recognized in Earnings (Detail) - Fixed maturities - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]      
Beginning balance $ 11,500,000 $ 12,800,000 $ 13,100,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment 400,000 0 0
Reductions for securities sold/matured (11,200,000) 0 (1,400,000)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows (200,000) (1,300,000) 1,100,000
Ending balance 500,000 11,500,000 12,800,000
Residential mortgage-backed securities      
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]      
Beginning balance 11,100,000 12,400,000 12,700,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment 0 0 0
Reductions for securities sold/matured (10,900,000) 0 (1,400,000)
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows (200,000) (1,300,000) 1,100,000
Ending balance 0 11,100,000 12,400,000
Commercial mortgage-backed securities      
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Roll Forward]      
Beginning balance 400,000 400,000 400,000
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Additions, No Previous Impairment 400,000 0 0
Reductions for securities sold/matured (300,000) 0 0
Other than Temporary Impairment, Credit Losses Recognized in Earnings, Changes in Cash Flows 0 0 0
Ending balance $ 500,000 $ 400,000 $ 400,000