Quarterly report pursuant to Section 13 or 15(d)

Property and Equipment - Additional Information (Detail)

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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