Investments (Contractual maturity) (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Cost or Amortized Cost
Due in one year or less, cost or amortized cost	$ 1,783	$ 1,648
Due after one year through five years, cost or amortized cost	12,916	13,603
Due after five years through ten years, cost or amortized cost	9,430	8,726
Due after ten years, cost or amortized cost	14,341	14,164
Total Amortized Cost Basis	38,470	38,141
Estimated Fair Value
Due in one year or less, estimated fair value	1,819	1,665
Due after one year through five years, estimated fair value	13,731	14,442
Due after five years through ten years, estimated fair value	10,267	9,555
Due after ten years, estimated fair value	16,919	16,942
Total Estimated Fair Value	$ 42,736	$ 42,604