Fair value of financial instruments - Additional Information (Detail)	3 Months Ended
	Mar. 31, 2012	Dec. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Discount rate, lower limit	0.24%	0.01%
Discount rate, upper limit	5.07%	4.17%
Number of business days in which overdrafts are generally repaid	2 days
Commercial paper maturity period	397 days