Investments (Contractual Maturity) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Investments [Abstract]
Amortized Cost, Due in one year or less	$ 1,802	$ 1,515
Amortized Cost, Due after one year through five years	13,110	11,198
Amortized Cost, Due after five years through ten years	8,410	10,034
Amortized Cost, Due after ten years	14,017	13,686
Amortized Cost, Total	37,339	36,433
Estimated Fair Value, Due in one year or less	1,812	1,506
Estimated Fair Value, Due after one year through five years	13,537	11,653
Estimated Fair Value, Due after five years through ten years	8,890	10,437
Estimated Fair Value, Due after ten years	15,692	13,987
Estimated Fair Value, Total	$ 39,931	$ 37,583