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Capital Stock - ATM Agreements - Additional Information (Details) - USD ($) | 1 Months Ended | 12 Months Ended
--- | --- | ---
Feb. 28, 2017 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Class Of Warrant Or Right [Line Items] |  |  |  |
Proceeds from issuance of common stock |  | $ 10,160,000 | $ 57,000 | $ 171,000
Common Stock [Member] |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |
Sale of shares | 1,200,000 | 1,849,000 | 13,000 |
2015 ATM Agreement [Member] |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |
Sales commission paid per transaction (in hundredths) |  | 3.00% |  |
Proceeds from issuance of common stock |  | $ 600,000 | $ 56,000 |
Offering costs offset against proceeds upon sale of common stock |  | $ 200,000 | $ 56,000 |
2015 ATM Agreement [Member] | Common Stock [Member] |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |
Sale of shares |  | 110,252 | 10,365 | 0

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