Commitments and contingent liabilities - Additional Information (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Minimum
Loss Contingencies [Line Items]
Estimated time frame over which accrued amounts may be paid out	10 years
Maximum
Loss Contingencies [Line Items]
Estimated time frame over which accrued amounts may be paid out	15 years
Accrued Liabilities
Loss Contingencies [Line Items]
Waste disposed accrued liability	$ 0.7	$ 0.8
Contingency reserve for litigation	0.4	0.6
Other Liability
Loss Contingencies [Line Items]
Waste disposed accrued liability	$ 6.2	$ 5.6