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Investments (Fixed Maturity Securities by Contractual Maturity) (Details) - USD ($)
$ in Thousands
Dec. 31, 2016
Dec. 31, 2015
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]    
Available for sale, due in one year or less, amortized cost $ 236,707  
Available for sale, due after one year through five years, amortized cost 2,824,776  
Available for sale, due after five years through ten years, amortized cost 11,659,762  
Available for sale, due after ten years through twenty years, amortized cost 8,792,470  
Available for sale, due after twenty years, amortized cost 8,055,686  
Available for sale, securities with a single maturity date, amortized cost 31,569,401  
Available for sale, amortized cost 39,953,955 $ 35,823,710
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Available for sale, due in one year or less, fair value 241,934  
Available for sale, due after one year through five years, fair value 2,979,768  
Available for sale, due after five years through ten years, fair value 11,738,156  
Available for sale, due after ten years through twenty years, fair value 9,284,726  
Available for sale, due after twenty years, fair value 8,389,717  
Available for sale, securities with a single maturity date, fair value 32,634,301  
Available for sale, fair value 41,060,494 36,421,839
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Held for investment, due in one year or less, amortized cost 0  
Held for investment, due after one year through five years, amortized cost 0  
Held for investment, due after five years through ten years, amortized cost 0  
Held for investment, due after ten years through twenty years, amortized cost 0  
Held for investment, due after twenty years, amortized cost 76,825  
Held for investment, securities with a single maturity date, amortized cost 76,825  
Held for investment, amortized cost 76,825 76,622
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]    
Held for investment, due in one year or less, fair value 0  
Held for investment, due after one year through five years, fair value 0  
Held for investment, due after five years through ten years, fair value 0  
Held for investment, due after ten years through twenty years, fair value 0  
Held for investment, due after twenty years, fair value 68,766  
Held for investment, securities with a single maturity date, fair value 68,766  
Held for investment, fair value 68,766 $ 65,377
Residential Mortgage Backed Securities [Member]    
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]    
Available for sale, securities without a single maturity date, amortized cost 1,166,944  
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Available for sale, securities without a single maturity date, fair value 1,254,835  
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Held for investment, securities without a single maturity date, amortized cost 0  
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]    
Held for investment, securities without a single maturity date, fair value 0  
Commercial Mortgage Backed Securities [Member]    
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]    
Available for sale, securities without a single maturity date, amortized cost 5,422,255  
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Available for sale, securities without a single maturity date, fair value 5,365,235  
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Held for investment, securities without a single maturity date, amortized cost 0  
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]    
Held for investment, securities without a single maturity date, fair value 0  
Other Asset Backed Securities [Member]    
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]    
Available for sale, securities without a single maturity date, amortized cost 1,795,355  
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]    
Available for sale, securities without a single maturity date, fair value 1,806,123  
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]    
Held for investment, securities without a single maturity date, amortized cost 0  
Held-to-maturity Securities, Debt Maturities, Fair Value [Abstract]    
Held for investment, securities without a single maturity date, fair value $ 0