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Capital Stock - ATM Agreements - Additional Information (Details) - USD ($) |  |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 18, 2018 | Nov. 12, 2015 | Dec. 31, 2018 | Feb. 28, 2017 | Dec. 31, 2018 | Dec. 31, 2017
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |
Proceeds from issuance of common stock |  |  |  |  | $ 6,063,000 | $ 10,160,000
Offering costs offset against proceeds upon sale of common stock |  |  | $ 300,000 |  |  |
Common Stock [Member] |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |
Sale of shares |  |  | 285,689 | 1,200,000 | 354,000 | 1,849,000
2018 ATM Agreement [Member] |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |
Sales commission paid per transaction (in hundredths) | 3.00% |  |  |  |  |
Maximum amount of shares can be issued | $ 30,000,000 |  |  |  |  |
2015 ATM Agreement [Member] |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |
Sales commission paid per transaction (in hundredths) |  | 3.00% |  |  |  |
Maximum amount of shares can be issued |  | $ 75,000,000 |  |  |  |
Proceeds from issuance of common stock |  |  |  |  | $ 2,400,000 | $ 600,000
Offering costs offset against proceeds upon sale of common stock |  |  |  |  | $ 100,000 | $ 200,000
Agreement expired date |  | 2018-02 |  |  |  |
2015 ATM Agreement [Member] | Common Stock [Member] |  |  |  |  |  |
Class Of Warrant Or Right [Line Items] |  |  |  |  |  |
Sale of shares |  |  |  |  | 67,658 | 110,252

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