Summary of Significant Accounting Policies (Details 4) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Property and equipment
Accumulated depreciation on property and equipment	$ 2,410,000,000	$ 2,290,000,000
Depreciation expense on property and equipment	$ 214,000,000	$ 222,000,000	$ 239,000,000
Real property
Property and equipment
Property and equipment, estimated useful life, average	40 years
Equipment | Minimum
Property and equipment
Property and equipment, estimated useful life, average	3 years
Equipment | Maximum
Property and equipment
Property and equipment, estimated useful life, average	10 years