Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost, Due in one year or less	$ 14,582
Amortized Cost, Due after one year through five years	71,452
Amortized Cost, Due after five years through ten years	70,413
Amortized Cost, Due after ten years	119,319
Amortized Cost, Structured Securities	62,264
Amortized Cost, Subtotal	338,030	$ 330,354
Estimated Fair Value, Due in one year or less	14,713
Estimated Fair Value, Due after one year through five years	74,867
Estimated Fair Value, Due after five years through ten years	74,093
Estimated Fair Value, Due after ten years	136,641
Estimated Fair Value, Structured Securities	63,770
Available-for-sale Securities, Debt Securities	$ 364,084	$ 350,889