Investments (Contractual maturity) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2014	Dec. 31, 2013
Investments [Abstract]
Due in one year or less, cost or amortized cost	$ 2,684	$ 2,420
Due after one year through five years, cost or amortized cost	8,839	9,496
Due after five years through ten years, cost or amortized cost	11,455	11,667
Due after ten years, cost or amortized cost	13,920	15,692
Total Amortized Cost Basis	36,898	39,275
Due in one year or less, estimated fair value	2,729	2,455
Due after one year through five years, estimated fair value	9,401	10,068
Due after five years through ten years, estimated fair value	11,876	11,954
Due after ten years, estimated fair value	15,394	16,720
Total Estimated Fair Value	$ 39,400	$ 41,197