Other Financial Data (Sigma Fund) (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Sigma Fund
Sigma Fund, Current	$ 1,931	$ 3,208
Cash [Member]
Sigma Fund
Sigma Fund, Current	15	264
U.S. Government, Agency and Government-sponsored Enterprise Obligations [Member]
Sigma Fund
Sigma Fund, Current	$ 1,916	$ 2,944