Fair value of financial instruments - Estimated Fair Value of Private Placement Warrants Publicly Traded Stock Pricing Approach (Detail) (USD $)	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Risk-free interest rate	0.92%
Estimated price volatility	55.00%
Private Placement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market value of the Company's common stock	$ 33.59	$ 16.18
Exercise price	13.00	13.00
Risk-free interest rate	0.61%	0.42%
Estimated price volatility	45.00%	45.00%
Marketability discount	10.00%	15.00%
Contractual term	2 years 9 months 29 days	3 years 3 months 29 days
Dividend yield