Stockholders' equity - Additional Information (Detail) - USD ($)	1 Months Ended						12 Months Ended
	Jan. 31, 2016	Jun. 11, 2014	Jun. 06, 2014	Mar. 07, 2011	Mar. 04, 2011	Jul. 07, 2010	Dec. 31, 2015	Dec. 31, 2014
Stockholders Equity [Line Items]
Common Stock, Shares, Issued							32,804,617	32,312,276
Common Stock, Par Or Stated Value Per Share							$ 0.01	$ 0.01
Proceeds From Issuance Of Common Stock							$ 460,000	$ 15,925,000
Investment Warrant Expiration Date							Jul. 07, 2016
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
Reimbursement Expense					$ 150,000
Proceeds from Issuance or Sale of Equity, Total						$ 4,900,000
Class of Warrant or Right, Exercise Price of Warrants or Rights					$ 0.60	$ 2.60
Adjustment of Warrants Granted for Services						$ 1,300,000
Class Of Warrant Or Right Issued Unit During Period				1,218,750
2010 Offering | Subsequent Event
Stockholders Equity [Line Items]
Class of Warrant or Right, Exercise Price of Warrants or Rights	$ 2.60
Class Of Warrant Or Right Issued Unit During Period	44,000
Underwritten Offering
Stockholders Equity [Line Items]
Common Stock, Shares, Issued			6,000,000
Common Stock, Par Or Stated Value Per Share			$ 0.01
Additional Offering Expenses		$ 400,000
Underwriters
Stockholders Equity [Line Items]
Proceeds From Issuance Of Common Stock		$ 16,128,750
Stock Issued During Period, Shares, New Issues		6,900,000	6,000,000
Share Price		$ 2.3375	$ 2.3375
Purchase Of Additional Common Stock		900,000	900,000
Reimbursement Expense		$ 150,000
Public
Stockholders Equity [Line Items]
Share Price			$ 2.50