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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, 2017
Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]    
Coupon 5.90% 5.90%
Yield 4.20% 6.00%
Weighted Average Life 2 years 2 years 6 months