Pension and postretirement benefits - Additional Information (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 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	$ 50
Defined Benefit Plan Contributions By Employer	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
Defined Benefit Plan Contributions By Employer	$ 1,600