Stock-based compensation (Details 2)	3 Months Ended	9 Months Ended
Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Employee Stock Options.
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average expected term (years)	5 years 7 months 6 days	5 years 9 months 18 days	5 years 4 months 24 days	5 years 6 months
Weighted-average volatility	63.00%	66.00%	64.00%	68.00%
Risk-free interest rate Minimum	1.63%	1.62%	1.37%	1.62%
Risk-free interest rate Maximum	1.85%	1.88%	1.85%	1.88%
Expected dividends	0.00%	0.00%	0.00%	0.00%
Stock Appreciation Units (SARs) [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average expected term (years)	3 years 6 months	2 years 6 months	3 years 7 months 6 days	2 years 6 months
Weighted-average volatility	60.00%	54.00%	62.00%	56.00%
Risk-free interest rate Minimum	0.28%	0.47%	0.25%	0.13%
Risk-free interest rate Maximum	1.38%	0.88%	1.57%	0.90%
Expected dividends	0.00%	0.00%	0.00%	0.00%
Employee Stock Purchase Plan [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average expected term (years)			8 months 12 days	8 months 12 days
Weighted-average volatility			58.00%	54.00%
Risk-free interest rate Minimum			0.03%	0.05%
Risk-free interest rate Maximum			0.14%	0.13%
Expected dividends	0.00%	0.00%	0.00%	0.00%

The estimated dividend rate (a percentage of the share price) to be paid (expected dividends) to holders of the underlying shares over the option's term.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(iii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

The maximum risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

No definition available.

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- Definition

The minimum risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

No definition available.

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- Definition

Weighted average expected volatility rate of share-based compensation awards.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(ii)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

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- Definition

Expected term of share-based compensation awards, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SAB TOPIC 14.D.2)

-URI http://asc.fasb.org/extlink&oid=27013229&loc=d3e301413-122809

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (f)(2)(i)

-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Staff Accounting Bulletin (SAB)

-Number Topic 14

-Section D

-Subsection 2

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