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Investments (Narrative) (Details)
$ in Thousands
12 Months Ended
Dec. 31, 2015
USD ($)
Securities
Dec. 31, 2014
USD ($)
Securities
Dec. 31, 2013
USD ($)
Investments [Abstract]      
Fixed income securities with call features, percentage 33.00%    
Fixed income securities with call features, currently callable, percentage 2.00%    
Fixed income securities with call features, currently callable, amount $ 700,000    
Fixed income securities with call features, callable during next 12 months, percentage 0.40%    
Fixed income securities with call features, callable during next 12 months, amount $ 200,000    
Fixed income securities with call features, not callable until within 6 months of stated maturity, percentage 68.00%    
Percentage of fixed maturity portfolio rated investment grade based on NAIC designations 98.00% 98.00%  
Number of individual securities with unrealized losses | Securities 1,246 402  
Percentage of unrealized losses on fixed maturity securities where securities are rated investment grade 84.00% 78.00%  
Assets held by insurance regulators $ 38,300,000 $ 32,600,000  
Investment Holdings [Line Items]      
Available for sale, amortized cost 35,823,710 30,205,046  
Principal repayments, calls and tenders of available for sale securities $ 1,612,121 1,490,906 $ 3,456,719
Stockholders' Equity, Total [Member]      
Investment Holdings [Line Items]      
Threshold for disclosure percentage 10.00%    
Investment Type Concentration Risk [Member]      
Investment Holdings [Line Items]      
Fair value, concentration of risk, investments $ 0 0  
Fixed Maturity Securities [Member]      
Investment Holdings [Line Items]      
Principal repayments, calls and tenders of available for sale securities 1,200,000 1,300,000 2,100,000
Available For Sale [Member]      
Investment Holdings [Line Items]      
Proceeds from sales of available for sale securities 400,000 200,000 $ 1,500,000
Available For Sale [Member] | Fixed Maturity Securities [Member]      
Investment Holdings [Line Items]      
Available for sale securities, fair value $ 36,421,839 32,445,202  
Residential Mortgage Backed Securities [Member] | Alt-A Sector [Member]      
Investment Holdings [Line Items]      
Number of securities in unrealized loss position | Securities 34    
Available for sale, amortized cost $ 204,700    
Available for sale securities, fair value 227,500    
Residential Mortgage Backed Securities [Member] | Available For Sale [Member] | Fixed Maturity Securities [Member]      
Investment Holdings [Line Items]      
Available for sale securities, fair value $ 1,462,072 $ 1,751,345