Investments (Contractual maturity) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 1,574	$ 2,479
Due after one year through five years, cost or amortized cost	7,721	9,054
Due after five years through ten years, cost or amortized cost	14,652	12,055
Due after ten years, cost or amortized cost	13,303	13,728
Total Amortized Cost Basis	37,250	37,316
Due in one year or less, estimated fair value	1,595	2,511
Due after one year through five years, estimated fair value	8,070	9,605
Due after five years through ten years, estimated fair value	14,915	12,584
Due after ten years, estimated fair value	14,989	16,049
Total Estimated Fair Value	$ 39,569	$ 40,749