Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,779	$ 1,574
Due after one year through five years, cost or amortized cost	7,566	7,721
Due after five years through ten years, cost or amortized cost	15,892	14,652
Due after ten years, cost or amortized cost	13,112	13,303
Total Amortized Cost Basis	38,349	37,250
Due in one year or less, estimated fair value	1,828	1,595
Due after one year through five years, estimated fair value	7,955	8,070
Due after five years through ten years, estimated fair value	16,332	14,915
Due after ten years, estimated fair value	14,778	14,989
Total Estimated Fair Value	$ 40,893	$ 39,569