Pension plans and other postretirement benefits - Amortization of Amounts in Accumulated Other Comprehensive Income Expected to be Recognized as Components of Net Periodic Benefit Expense (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012
Pension benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Amortization of net prior service cost (credit)	$ (6,559)
Amortization of net loss	41,593
Other postretirement benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Amortization of net prior service cost (credit)	(1,356)
Amortization of net loss	$ 347