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Debt Securities - Overall Statistics for Company's AAA-Rated CMBS Investments Calculated on Weighted Average Basis (Detail) - Commercial Mortgage Backed Securities [Member]
9 Months Ended 12 Months Ended
Sep. 30, 2016
Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]    
Coupon 5.90% 5.90%
Yield 5.90% 6.50%
RemainingWeighted Average Life 2 years 1 year 7 months 6 days