Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost, Due in one year or less	$ 1,730
Amortized Cost, Due after one year through five years	8,497
Amortized Cost, Due after five years through ten years	11,642
Amortized Cost, Due after ten years	23,340
Amortized Cost, Structured Securities	13,251
Amortized Cost, Subtotal	58,460	$ 58,599
Estimated Fair Value, Due in one year or less	1,733
Estimated Fair Value, Due after one year through five years	8,734
Estimated Fair Value, Due after five years through ten years	11,702
Estimated Fair Value, Due after ten years	26,063
Estimated Fair Value, Structured Securities	13,328
Available-for-sale Securities, Debt Securities	$ 61,560	$ 63,333