Accounts Receivable and Major Customers - Additional Information (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018 USD ($) Customer	Mar. 31, 2017 USD ($) Customer	Dec. 31, 2017 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Document Period End Date	Mar. 31, 2018
Sales Revenue | Customer Concentration Risk [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of major customers | Customer	3	1
Revenues, major customers	$ 70.6	$ 31.2
Concentration risk, percentage	91.00%	90.00%
Sales Revenue | Customer Concentration Risk [Member] | Major Customer 1
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues, major customers	$ 31.0
Concentration risk, percentage	40.00%
Sales Revenue | Customer Concentration Risk [Member] | Major Customer 2
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues, major customers	$ 26.4
Concentration risk, percentage	34.00%
Sales Revenue | Customer Concentration Risk [Member] | Major Customer Three [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Revenues, major customers	$ 13.2
Concentration risk, percentage	17.00%
Accounts Receivable | Credit Concentration Risk
Accounts, Notes, Loans and Financing Receivable [Line Items]
Concentration risk, percentage	89.00%		82.00%
Accounts receivable, major customers	$ 38.8		$ 32.1