Investments (Fixed Maturity Securities by Contractual Maturity) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis [Abstract]
Available for sale, due in one year or less, amortized cost	$ 197,449
Available for sale, due after one year through five years, amortized cost	4,958,029
Available for sale, due after five years through ten years, amortized cost	10,556,868
Available for sale, due after ten years through twenty years, amortized cost	9,214,395
Available for sale, due after twenty years, amortized cost	8,553,637
Available for sale, securities with a single maturity date, amortized cost	33,480,378
Available for sale, amortized cost	43,116,759	$ 39,953,955
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available for sale, due in one year or less, fair value	201,079
Available for sale, due after one year through five years, fair value	5,114,727
Available for sale, due after five years through ten years, fair value	10,827,764
Available for sale, due after ten years through twenty years, fair value	10,080,377
Available for sale, due after twenty years, fair value	9,378,089
Available for sale, securities with a single maturity date, fair value	35,602,036
Available for sale, fair value	45,372,989	41,060,494
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	77,041
Held for investment, securities with a single maturity date, amortized cost	77,041
Held for investment, amortized cost	77,041	76,825
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	76,460
Held for investment, securities with a single maturity date, fair value	76,460
Held for investment, fair value	76,460	$ 68,766
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	1,028,484
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available for sale, securities without a single maturity date, fair value	1,105,567
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	5,531,922
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available for sale, securities without a single maturity date, fair value	5,544,850
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	3,075,975
Available-for-sale Securities, Debt Maturities, Fair Value [Abstract]
Available for sale, securities without a single maturity date, fair value	3,120,536
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