Commitments and contingencies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments And Contingencies [Line Items]
Operating Leases, Rent Expense	$ 1,700,000	$ 1,400,000	$ 1,200,000
Accrual for Environmental Loss Contingencies	$ 90,000