Investments (Maturities of Fixed Maturity Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016
Available-for-sale Securities, Debt Maturities [Abstract]
Amortized Cost, Due in one year or less	$ 15,792
Amortized Cost, Due after one year through five years	69,241
Amortized Cost, Due after five years through ten years	67,938
Amortized Cost, Due after ten years	119,394
Amortized Cost, Structured Securities	61,620
Amortized Cost, Subtotal	333,985	$ 330,354
Estimated Fair Value, Due in one year or less	15,902
Estimated Fair Value, Due after one year through five years	72,487
Estimated Fair Value, Due after five years through ten years	70,860
Estimated Fair Value, Due after ten years	133,672
Estimated Fair Value, Structured Securities	62,607
Available-for-sale Securities, Debt Securities	$ 355,528	$ 350,889