Concentration of risk (Details)	3 Months Ended			12 Months Ended
	Mar. 31, 2012	Dec. 31, 2012 Accounts receivable Concentration of credit risk item	Dec. 31, 2011 Accounts receivable Concentration of credit risk item	Dec. 31, 2012 Accounts receivable Concentration of credit risk Customer with largest individual trade balance	Dec. 31, 2011 Accounts receivable Concentration of credit risk Customer with largest individual trade balance	Dec. 31, 2012 Accounts receivable Concentration of credit risk Customer with second largest individual trade balance	Dec. 31, 2011 Accounts receivable Concentration of credit risk Customer with second largest individual trade balance	Dec. 31, 2011 Accounts receivable Concentration of credit risk Customer with third largest individual trade balance	Dec. 31, 2012 U.S. workforce Labor concentration risk
Concentration of risks
Concentration risk, percentage				55.00%	53.00%	25.00%	15.00%	10.00%	68.00%
Period of new collective bargaining agreement	2 years
Number of largest individual trade receivable balances		2	3