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Net Loss Per Share (Details) - USD ($)
3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 21, 2016
Earnings Per Share [Abstract]          
Weighted-average common shares outstanding (in shares) 150,927,825 130,436,038 150,128,204 112,819,041  
Securities Financing Transaction [Line Items]          
Fees and issuance costs     $ 638,000 $ 1,340,000  
Citigroup Global Markets Inc. | Equity Distribution Agreement          
Securities Financing Transaction [Line Items]          
Equity financing agreement, amount authorized (up to)         $ 200,000,000
Gross proceeds $ 0 $ 16,066,000 10,767,000 69,113,000  
Fees and issuance costs 0 386,000 311,000 1,728,000  
Net proceeds $ 0 $ 15,680,000 $ 10,456,000 $ 67,385,000  
Shares issued (in shares) 0 3,824,144 3,802,500 21,325,587