Pension and postretirement benefits - Additional information (Detail) (USD $)	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2013
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	$ 51,000	$ 51,000
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	6,800,000	6,800,000
Defined Benefit Plan Contributions By Employer	$ 1,800,000	$ 1,800,000