Fair value of financial instruments - Additional Information (Detail) (USD $)			3 Months Ended		6 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2013 Private Placement Warrants [Member]	Jun. 30, 2012 Private Placement Warrants [Member]	Jun. 30, 2013 Private Placement Warrants [Member]	Jun. 30, 2012 Private Placement Warrants [Member]	Jun. 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	10.00%	15.00%
Warrant sensitivity percentage							10.00%
Estimated fair value			$ 4,207,000	$ (1,050,000)	$ 9,053,000	$ (457,000)	$ 992,000