Property and Equipment - Additional Information (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Property, Plant and Equipment [Line Items]
Gross property and equipment	$ 20,206		$ 15,636
Depreciation expense	156	$ 133
Asset Under Construction [Member]
Property, Plant and Equipment [Line Items]
Gross property and equipment	7,988		3,671
Assets Under Construction Held Under Finance Lease [Member]
Property, Plant and Equipment [Line Items]
Gross property and equipment	$ 7,600		$ 3,100