Annual report pursuant to Section 13 and 15(d)

Property and Equipment - Narrative (Details)

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Property and Equipment - Narrative (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Abstract]      
Depreciation $ 8,818 $ 8,160 $ 10,066