Commitments and contingencies - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Commitments And Contingencies [Line Items]
Operating Leases, Rent Expense	$ 1,400,000	$ 1,200,000	$ 800,000
Remediation Costs Incurred	0	$ 0	$ 53,000
Accrual for Environmental Loss Contingencies	$ 100,000