Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,135	$ 1,779
Due after one year through five years, cost or amortized cost	8,165	7,566
Due after five years through ten years, cost or amortized cost	16,060	15,892
Due after ten years, cost or amortized cost	12,852	13,112
Total Amortized Cost Basis	38,212	38,349
Due in one year or less, estimated fair value	1,157	1,828
Due after one year through five years, estimated fair value	8,501	7,955
Due after five years through ten years, estimated fair value	16,718	16,332
Due after ten years, estimated fair value	15,108	14,778
Total Estimated Fair Value	$ 41,484	$ 40,893