Employee benefit plans - Summary of Actuarial Assumptions Used to Determine Net Periodic Benefit Cost (Credit) (Detail)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
U.S. defined benefit pension plans [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	4.20%	5.10%	5.70%
Long-term return on plan assets	10.00%	10.00%	10.00%
Foreign Pension Plan Defined Benefit [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	4.90%	5.10%	5.50%
Increase in future compensation levels	3.10%	3.10%	3.00%
Long-term return on plan assets	5.20%	5.50%	5.50%