Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost, Due in one year or less	$ 1,833
Amortized Cost, Due after one year through five years	10,018
Amortized Cost, Due after five years through ten years	11,131
Amortized Cost, Due after ten years	22,991
Amortized Cost, Structured Securities	12,626
Amortized Cost, Subtotal	58,599	$ 57,289
Estimated Fair Value, Due in one year or less	1,838
Estimated Fair Value, Due after one year through five years	10,347
Estimated Fair Value, Due after five years through ten years	11,458
Estimated Fair Value, Due after ten years	26,786
Estimated Fair Value, Structured Securities	12,904
Available-for-sale Securities, Debt Securities	$ 63,333	$ 59,899