Pension and postretirement benefits - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012
Pensions And Benefit Restoration Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan Estimated Future Employer Contributions In Current Fiscal Year	$ 58	$ 58
Defined Benefit Plan Contributions By Employer	0	0
Other Postretirement Benefit Plans Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plan Estimated Future Employer Contributions In Current Fiscal Year	7,400	7,400
Defined Benefit Plan Contributions By Employer	$ 1,800	$ 5,600