Other Financial Data (Sigma Fund) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Sigma Fund
Sigma Fund, Current	$ 3,208	$ 4,646
Sigma Fund, Non-Current	0	70
Cash [Member]
Sigma Fund
Sigma Fund, Current	264	2,355
Sigma Fund, Non-Current		0
U.S. Government And Agency Obligations [Member]
Sigma Fund
Sigma Fund, Current	2,944	2,291
Sigma Fund, Non-Current		0
Corporate Bonds [Member]
Sigma Fund
Sigma Fund, Current
Sigma Fund, Non-Current	0	58
Asset-Backed Bonds [Member]
Sigma Fund
Sigma Fund, Current
Sigma Fund, Non-Current	0	1
Mortgage-Backed Bonds [Member]
Sigma Fund
Sigma Fund, Current	0
Sigma Fund, Non-Current	$ 0	$ 11