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Stockholders' equity - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands |  |  | 1 Months Ended | 12 Months Ended
--- | --- | --- | --- | ---
Dec. 14, 2016 | Dec. 08, 2016 | Jul. 31, 2016 | Jan. 31, 2016 | Jun. 11, 2014 | Mar. 07, 2011 | Mar. 04, 2011 | Jul. 07, 2010 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |
Common Stock, Shares, Issued |  |  |  |  |  |  |  |  | 42,398,140 | 41,465,820 |
Common Stock, Par Or Stated Value Per Share |  |  |  |  |  |  |  |  | $ 0.01 | $ 0.01 |
Proceeds From Issuance Of Common Stock |  |  |  |  |  |  |  |  | $ 807 | $ 51,060 | $ 460
Common Stock |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |
Common Stock, Shares, Issued |  |  |  |  |  |  |  | 2,737,500 |  |  |
Stock Unit |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |
Common Stock, Shares, Issued |  |  |  |  |  |  |  | 2,737,500 |  |  |
2010 Offering |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |
Common Stock, Shares, Issued |  |  |  |  |  |  |  | 1,368,750 |  |  |
Common Stock, Par Or Stated Value Per Share |  |  |  |  |  |  |  | $ 2.00 |  |  |
Proceeds from Issuance or Sale of Equity, Total |  |  |  |  |  |  |  | $ 4,900 |  |  |
Class of Warrant or Right, Exercise Price of Warrants or Rights |  |  |  | $ 2.60 |  |  | $ 0.60 | $ 2.60 |  |  |
Adjustment of Warrants Granted for Services |  |  |  |  |  |  |  | $ 1,300 |  |  |
Class Of Warrant Or Right Repurchase During Period |  |  |  |  |  |  | 150,000 |  |  |  |
Class Of Warrant Or Right Issued Unit During Period |  |  | 7,498 | 1,161,252 |  | 1,218,750 |  |  |  |  |
Underwritten Offering |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |
Common Stock, Shares, Issued |  | 6,428,571 |  |  |  |  |  |  |  |  |
Common Stock, Par Or Stated Value Per Share |  | $ 0.01 |  |  |  |  |  |  |  |  |
Proceeds From Issuance Of Common Stock | $ 48,400 |  |  |  |  |  |  |  |  |  |
Additional Offering Expenses | 3,300 |  |  |  |  |  |  |  |  |  |
Underwriters |  |  |  |  |  |  |  |  |  |  |
Stockholders Equity [Line Items] |  |  |  |  |  |  |  |  |  |  |
Proceeds From Issuance Of Common Stock | $ 51,700 |  |  |  |  |  |  |  |  |  |
Stock Issued During Period, Shares, New Issues |  | 6,428,571 |  |  | 7,392,856 |  |  |  |  |  |
Share Price | $ 7.00 |  |  |  |  |  |  |  |  |  |
Purchase Of Additional Common Stock |  | 964,285 |  |  | 964,285 |  |  |  |  |  |

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