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