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Securities - Percentage Vintage, Property Type, and Location of Collateral Securing Company's AAA-Rated CMBS Investments Calculated on Weighted Average Basis (Detail) - Available-for-sale Securities [Member]
6 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Vintage Concentration Risk [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 100.00% 100.00%
Vintage Concentration Risk [Member] | Vintage 2005 [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 0.90% 2.00%
Vintage Concentration Risk [Member] | Vintage 2006 [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 15.40% 12.10%
Vintage Concentration Risk [Member] | Vintage 2007 [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 68.00% 73.50%
Vintage Concentration Risk [Member] | Vintage 2008 [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 15.70% 12.40%
Product Concentration Risk [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 100.00% 100.00%
Product Concentration Risk [Member] | Office [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 35.80% 34.60%
Product Concentration Risk [Member] | Retail [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 30.60% 29.00%
Product Concentration Risk [Member] | Multifamily [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 12.40% 12.40%
Product Concentration Risk [Member] | Other    
Mortgage Loans on Real Estate [Line Items]    
Percentage 21.20% 24.00%
Geographic Concentration Risk [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 100.00% 100.00%
Geographic Concentration Risk [Member] | South Atlantic [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 26.90% 23.80%
Geographic Concentration Risk [Member] | Middle Atlantic [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 16.50% 16.70%
Geographic Concentration Risk [Member] | Pacific [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 14.30% 15.30%
Geographic Concentration Risk [Member] | East North Central [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 14.40% 10.80%
Geographic Concentration Risk [Member] | Other [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 27.90% 33.40%