Investments (Contractual maturity) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Cost or Amortized Cost
Due in one year or less, cost or amortized cost	$ 1,802	$ 1,515
Due after one year through five years, cost or amortized cost	13,110	11,198
Due after five years through ten years, cost or amortized cost	8,410	10,022
Due after ten years, cost or amortized cost	14,017	13,686
Total Amortized Cost Basis	37,339	36,421
Estimated Fair Value
Due in one year or less, estimated fair value	1,812	1,506
Due after one year through five years, estimated fair value	13,537	11,653
Due after five years through ten years, estimated fair value	8,890	10,425
Due after ten years, estimated fair value	15,692	13,987
Total Estimated Fair Value	$ 39,931	$ 37,571