Fair value of financial instruments - Fair Value of Private Placement Warrants Publicly Traded Stock Pricing Approach (Detail) (USD $)	1 Months Ended	3 Months Ended	12 Months Ended
Jun. 30, 2012	Mar. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market value of the Company's common stock			$ 16.18	$ 9.55
Risk-free interest rate	0.92%
Estimated price volatility		55.00%	55.00%
Dividend yield			0.00%
Private Placement [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Market value of the Company's common stock		$ 26.05	$ 16.18
Exercise price		13.00	13.00
Risk-free interest rate		0.38%	0.42%
Estimated price volatility		45.00%	45.00%
Marketability discount		15.00%	15.00%
Contractual term		3 years 29 days	3 years 3 months 29 days
Dividend yield		0.00%	0.00%

Discounted valuation of stock.

No definition available.

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- Definition

Warrant contractual term.

+ References

No definition available.

+ Details

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- Definition

The exercise price of each class of warrants or rights outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 235

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.4-08.(i)(4))

-URI http://asc.fasb.org/extlink&oid=6881521&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 08

-Paragraph i

-Subparagraph 4

-Article 4

+ Details

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- Definition

Expected dividends to be paid to holders of the underlying shares or financial instruments (expressed as a percentage of the share or instrument's price).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

+ Details

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- Definition

Measure of dispersion, in percentage terms (for instance, the standard deviation or variance), for a given stock price.

+ References

+ Details

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- Definition

Risk-free interest rate assumption used in valuing an instrument.

+ References

+ Details

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- Definition

Price of a single share of a number of saleable stocks of a company.

+ References

+ Details

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