Employee separation charges (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended	12 Months Ended
	Jun. 30, 2012	Mar. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Employee separation charges [Abstract]
Defined Benefit Plan, Other Costs		$ (6)
Employee Separation Charges [Roll Forward]
Liability balance at beginning of period		90	90	22
Increase in liability (separation charges)	9		41	112
Reduction in liability (payments and other adjustments)			(117)	(44)
Liability balance at end of period	$ 14		$ 14	$ 90