Note Pension and postretirement benefits - Additional information (Detail)	3 Months Ended
	Mar. 31, 2017 USD ($)
Other Pension Plans Defined Benefit Member
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plans Estimated Future Employer Contributions In Current Fiscal Year	$ 236,000
Defined Benefit Plan Contributions By Employer	59,000
Other Postretirement Benefit Plans Defined Benefit [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Defined Benefit Plans Estimated Future Employer Contributions In Current Fiscal Year	6,400,000
Defined Benefit Plan Contributions By Employer	$ 1,500,000