Pension plans and other postretirement benefits - Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss (gain)	$ 43,136	$ 272,260
Prior service cost (credit)	(13,313)
Amortization of prior service (cost) credit	6,538	6,452
Amortization of loss	(37,916)	(20,566)
Total recognized in other comprehensive income, pre-tax	(1,555)	258,146
Pension benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss (gain)	43,318	261,957
Prior service cost (credit)
Amortization of prior service (cost) credit	6,559	6,559
Amortization of loss	(37,386)	(20,530)	(13,551)
Total recognized in other comprehensive income, pre-tax	12,491	247,986
Other postretirement benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Net loss (gain)	(182)	10,303
Prior service cost (credit)	(13,313)
Amortization of prior service (cost) credit	(21)	(107)
Amortization of loss	(530)	(36)	9
Total recognized in other comprehensive income, pre-tax	$ (14,046)	$ 10,160