Stock-based compensation - Estimated fair vale of stock-based awards (Details)	3 Months Ended	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Employee Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Weighted-average expected term (years)	5 years 10 months 24 days	5 years 7 months 6 days	5 years 9 months 18 days	5 years 4 months 24 days
Weighted-average volatility	65.00%	63.00%	65.00%	64.00%
Risk-free interest rate Minimum	1.01%	1.63%	1.01%	1.37%
Risk-free interest rate Maximum	1.15%	1.85%	1.76%	1.65%
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)	2 years 7 months 6 days	3 years 6 months	2 years 9 months 18 days	3 years 7 months 6 days
Weighted-average volatility	62.00%	60.00%	62.00%	62.00%
Risk-free interest rate Minimum	0.45%	0.28%	0.45%	0.25%
Risk-free interest rate Maximum	0.71%	1.38%	1.47%	1.57%
Expected dividends	0.00%	0.00%	0.00%	0.00%
Employee Stock Purchase Plan
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			70.00%	58.00%
Risk-free interest rate Minimum			0.08%	0.03%
Risk-free interest rate Maximum			0.39%	0.14%
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.

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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.

+ Details

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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.

+ Details

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- Definition

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

+ References

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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

No definition available.

+ Details

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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 50
-Paragraph 2
-Subparagraph (f)(2)(i)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

Reference 2: 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

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