Fair value of financial instruments - Additional Information (Detail) (USD $)			3 Months Ended		9 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2013 Private Placement Warrants [Member]	Sep. 30, 2012 Private Placement Warrants [Member]	Sep. 30, 2013 Private Placement Warrants [Member]	Sep. 30, 2012 Private Placement Warrants [Member]	Sep. 30, 2013 Private Placement Warrants [Member] Black Scholes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Discounted valuation of warrant	0.00%	15.00%
Warrant sensitivity percentage							10.00%
Estimated fair value			$ 12,605,000	$ (199,000)	$ 21,658,000	$ (656,000)	$ 2,000,000