Note Pension and postretirement benefits - Additional information (Detail) $ in Millions	9 Months Ended
	Sep. 30, 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	$ 16.2
Defined Benefit Plan Contributions By Employer	16.1
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	6.4
Defined Benefit Plan Contributions By Employer	$ 1.4