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Consolidated Statements of Stockholders' Equity (Parenthetical) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Statement of Stockholders' Equity [Abstract]      
Issuance of common stock for cash, financing costs $ 4,900 $ 128 $ 4,000