Stock-based compensation - Estimated fair vale of stock-based awards (Details)	3 Months Ended	9 Months Ended
Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average expected term (years)	6 years	5 years 10 months 24 days	6 years	5 years 9 months 18 days
Weighted-average volatility	65.00%	65.00%	65.00%	65.00%
Risk-free interest rate	2.02%
Risk-free interest rate minimum		1.01%	2.02%	1.01%
Risk-free interest rate maximum		1.15%	2.08%	1.76%
Expected dividends	0.00%	0.00%	0.00%	0.00%
Stock Appreciation Units (SAUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average expected term (years)	2 years 2 months 12 days	2 years 7 months 6 days	2 years 3 months 18 days	2 years 9 months 18 days
Weighted-average volatility	71.00%	62.00%	71.00%	62.00%
Risk-free interest rate minimum	1.03%	0.45%	0.51%	0.45%
Risk-free interest rate maximum	1.55%	0.71%	1.55%	1.47%
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)	0 years	0 years	8 months 1 day	8 months 12 days
Weighted-average volatility			55.00%	70.00%
Risk-free interest rate	0.00%	0.00%
Risk-free interest rate minimum			0.45%	0.08%
Risk-free interest rate maximum			0.91%	0.39%
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 risk-free interest rate assumption that is used in valuing an option on its own shares.

+ References

+ Details

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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.

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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.

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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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