Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Mar. 31, 2016	Dec. 31, 2015
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,747	$ 1,574
Due after one year through five years, cost or amortized cost	8,335	7,721
Due after five years through ten years, cost or amortized cost	14,587	14,652
Due after ten years, cost or amortized cost	12,600	13,303
Total Amortized Cost Basis	37,269	37,250
Due in one year or less, estimated fair value	1,773	1,595
Due after one year through five years, estimated fair value	8,748	8,070
Due after five years through ten years, estimated fair value	15,129	14,915
Due after ten years, estimated fair value	14,600	14,989
Total Estimated Fair Value	$ 40,250	$ 39,569