Commitments and contingent liabilities (Detail) $ in Millions	9 Months Ended
	Sep. 30, 2018 USD ($)
Loss Contingencies [Line Items]
Estimated litigation liability	$ 1.8
Minimum
Loss Contingencies [Line Items]
Estimated time fram over which accrued amounts may be paid out	15 years
Maximum
Loss Contingencies [Line Items]
Estimated time fram over which accrued amounts may be paid out	20 years
Accrued Liabilities
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 1.0
Other Liability
Loss Contingencies [Line Items]
Accrual for environmental loss contingencies	$ 5.5