Employee separation charges (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended	12 Months Ended
Mar. 31, 2012	Dec. 31, 2011
Employee separation charges
Net gains for pension and other postretirement benefit plans due to severance activities	$ 6
Separation Activity
Liability balance at beginning of period	90	22
Increase in liability (separation charges)	32	112
Reduction in liability (payments and other adjustments)	(103)	(44)
Liability balance at end of period	$ 19	$ 90

The charge against earnings in the period for known and estimated costs of termination benefits provided to current employees that are involuntarily terminated under a benefit arrangement associated with exit from or disposal of business activities or restructurings pursuant to a duly authorized plan, excluding costs or losses pertaining to an entity newly acquired in a business combination or a discontinued operation as defined by generally accepted accounting principles and costs associated with one-time termination benefits.

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+ Details