Summary of significant accounting policies (Details 3) (USD $) In Billions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Reporting Units
Definite-lived And Indefinite-lived Intangible Assets [Line Items]
Estimated fair value in excess of carrying value, low end of the range (in percent)	25.00%
Discount rate assumed to determine the fair value, low end of range (in percent)	10.50%
Discount rate assumed to determine the fair value, high end of the range (in percent)	10.50%
Acquired trade names
Definite-lived And Indefinite-lived Intangible Assets [Line Items]
Discount rate assumed to determine the fair value, low end of range (in percent)	10.50%
Royalty saving rate assumed to determine the fair value (in percent)	1.50%
Number of trade names that would have an impairment charge in the event of a one percentage point increase in the discount rate	0
Goodwill
Definite-lived And Indefinite-lived Intangible Assets [Line Items]
Estimated fair value in excess of carrying value, low end of the range	3