Pension and postretirement benefits - Additional information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2014
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
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	6,200,000
Defined Benefit Plan Contributions By Employer	$ 1,400,000