Commitments and contingencies - Additional Information (Detail) (USD $)	6 Months Ended	1 Months Ended
	Jun. 30, 2013	Jul. 05, 2013 Scenario, Forecast [Member]
Commitment And Contingencies [Line Items]
Registration Rights Agreement filing description	The Registration Rights Agreement provides among other things that if after such registration statement has been declared effective by the SEC, sales cannot be made pursuant to such registration statement for any reason, but excluding any period for which the use of any prospectus included in a registration statement has been suspended if and so long as certain conditions exist (which period may not be for more than 20 consecutive days or for a total of more than 45 days in any 12-month period), then the Company is required to pay amounts representing liquidated damages to each of the investors, on the same basis as the liquidated damages the Company could have been required to pay in connection with the initial registration statement filed pursuant to the Registration Rights Agreement.
Number of consecutive days for suspension of registration statement	20 days
Aggregated number of days, in which conditions regarding suspension persist	45 days
Number of months as base period for aggregate and consecutive period	12 months
Commitment towards investments in joint venture	$ 1,200,000
Contribution to joint venture		$ 500,000