Pension plans and other postretirement benefits - Assumed Weighted-Average Rates Used to Determine Net Benefit Expense (Detail)	1 Months Ended	12 Months Ended
	May 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Pension benefits [Member]	Dec. 31, 2011 Pension benefits [Member]	Dec. 31, 2010 Pension benefits [Member]	Dec. 31, 2012 Other postretirement benefits [Member]	Dec. 31, 2011 Other postretirement benefits [Member]	Dec. 31, 2010 Other postretirement benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Discount rate	5.25%	3.75%	4.25%	4.25%	5.25%	5.75%	4.25%	5.25%	5.75%
Long-term rate of return on plan assets				6.50%	6.50%	6.50%
Rate of increase in future compensation levels	4.00%			4.50%	4.50%	4.50%