Summary of significant accounting policies (Details) (USD $)			3 Months Ended		9 Months Ended
	Sep. 30, 2012	Dec. 31, 2011	Sep. 30, 2012 Other operating expense [Member]	Sep. 30, 2011 Other operating expense [Member]	Sep. 30, 2012 Other operating expense [Member]	Sep. 30, 2011 Other operating expense [Member]
Component of Operating Other Cost and Expense [Line Items]
Cost savings initiative charges			$ 36,000,000	$ 68,000,000	$ 106,000,000	$ 79,000,000
Summary of significant accounting policies (Textual) [Abstract]
Accumulated depreciation and amortization on property, plant and equipment	$ 6,400,000,000	$ 5,800,000,000