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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]
3 Months Ended 12 Months Ended
Mar. 31, 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 2.50% 2.00%
Vintage Concentration Risk [Member] | Vintage 2006 [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 14.90% 12.10%
Vintage Concentration Risk [Member] | Vintage 2007 [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 67.40% 73.50%
Vintage Concentration Risk [Member] | Vintage 2008 [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 15.20% 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 31.70% 34.60%
Product Concentration Risk [Member] | Retail [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 32.50% 29.00%
Product Concentration Risk [Member] | Multifamily [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 11.30% 12.40%
Product Concentration Risk [Member] | Other    
Mortgage Loans on Real Estate [Line Items]    
Percentage 24.50% 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 26.70% 23.80%
Geographic Concentration Risk [Member] | Middle Atlantic [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 15.70% 16.70%
Geographic Concentration Risk [Member] | Pacific [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 13.90% 15.30%
Geographic Concentration Risk [Member] | East North Central [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 12.10% 10.80%
Geographic Concentration Risk [Member] | Other [Member]    
Mortgage Loans on Real Estate [Line Items]    
Percentage 31.60% 33.40%