Stockholders' equity - Additional Information (Detail) (USD $)	12 Months Ended		1 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Jun. 11, 2014	Jun. 06, 2014	Mar. 07, 2011	Mar. 04, 2011	Jul. 07, 2010
Stockholders Equity [Line Items]
Common Stock, Shares, Issued	32,312,276	25,070,386
Common Stock, Par Or Stated Value Per Share	$ 0.01	$ 0.01
Investment Warrants Expiration Date	Jul. 07, 2016
Proceeds From Issuance Of Common Stock	$ 15,925,000	$ 1,110,000
Underwritten Offering [Member]
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 [Member]
Stockholders Equity [Line Items]
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
Proceeds From Issuance Of Common Stock			16,128,750
Reimbursement Expense			150,000
Public [Member]
Stockholders Equity [Line Items]
Share Price				$ 2.50
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							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 Repurchase During Period						150,000
Class Of Warrant Or Right Issued Unit During Period					1,218,750
Common Stock
Stockholders Equity [Line Items]
Common Stock, Shares, Issued							2,737,500
Stock Issued During Period, Shares, New Issues	6,900,000