Investments (Contractual maturity) (Details) (USD $) In Millions, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Cost or Amortized Cost
Due in one year or less	$ 2,475	$ 1,648
Due after one year through five years	10,687	13,603
Due after five years through ten years	10,586	8,726
Due after ten years	15,338	14,164
Total	39,086	38,141
Estimated Fair Value
Due in one year or less	2,516	1,665
Due after one year through five years	11,304	14,442
Due after five years through ten years	10,901	9,555
Due after ten years	16,510	16,942
Total	$ 41,231	$ 42,604