Annual report pursuant to Section 13 and 15(d)

Summary of Significant Accounting Policies - Property and Equipment, Impairment (Details)

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Summary of Significant Accounting Policies - Property and Equipment, Impairment (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Equipment and Impairment of Long-Lived Assets      
Depreciation $ 1,527 $ 1,273 $ 602
Impairment losses 0 $ 0 $ 0
Construction in progress      
Property, Equipment and Impairment of Long-Lived Assets      
Depreciation $ 0