Investments (Contractual maturity) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Cost or Amortized Cost
Due in one year or less, cost or amortized cost	$ 1,648	$ 1,802
Due after one year through five years, cost or amortized cost	13,603	13,110
Due after five years through ten years, cost or amortized cost	8,726	8,410
Due after ten years, cost or amortized cost	14,164	14,017
Total Amortized Cost Basis	38,141	37,339
Estimated Fair Value
Due in one year or less, estimated fair value	1,665	1,812
Due after one year through five years, estimated fair value	14,442	13,537
Due after five years through ten years, estimated fair value	9,555	8,890
Due after ten years, estimated fair value	16,942	15,692
Total Estimated Fair Value	$ 42,604	$ 39,931