Accounts Receivable and Major Customers - Additional Information (Detail) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2017 USD ($) Customer	Sep. 30, 2016 USD ($) Customer	Dec. 31, 2016 USD ($)
Sales Revenue | Customer Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of major customers | Customer	2	3
Revenues, major customers	$ 85.9	$ 93.5
Concentration risk, percentage	82.00%	94.00%
Sales Revenue | Customer Concentration Risk | Major Customer 1
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues, major customers	$ 72.6	$ 64.1
Concentration risk, percentage	69.00%	64.00%
Sales Revenue | Customer Concentration Risk | Major Customer 2
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues, major customers	$ 13.3	$ 15.8
Concentration risk, percentage	13.00%	16.00%
Sales Revenue | Customer Concentration Risk | Major Customer 3
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues, major customers		$ 13.6
Concentration risk, percentage		14.00%
Accounts Receivable | Credit Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	47.00%		81.00%
Accounts receivable, major customers	$ 14.6		$ 16.7