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Condensed consolidated balance sheets (Parenthetical) - $ / shares
Mar. 31, 2018
Dec. 31, 2017
Mar. 31, 2017
Statement of Financial Position [Abstract]      
Common stock, par value (in USD per share) $ 0.01 $ 0.01 $ 0.01
Common stock, shares authorized (in shares) 250,000,000 250,000,000 250,000,000
Common stock, shares issued (in shares) 47,425,139 46,617,830 45,655,937
Common stock, shares outstanding (in shares) 47,425,139 46,617,830 45,655,937