Stock-based compensation - Valuation assumptions of stock-based awards (Details)	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018	Dec. 31, 2018
Employee stock options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average expected term (years)	6 years	6 years 6 days	6 years	6 years 6 days
Weighted-average volatility	67.00%	66.00%	67.00%	65.00%
Risk-free interest rate	2.27%	2.62%	2.27%
Risk-free interest rate minimum				2.27%
Risk-free interest rate maximum				2.62%
Expected dividends	0.00%	0.00%	0.00%	0.00%
Stock appreciation units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average expected term (years)	1 year 6 months 18 days	2 years	1 year 7 months 10 days	2 years 15 days
Weighted-average volatility	62.00%	67.00%	63.00%	67.00%
Risk-free interest rate minimum	2.27%	1.73%	2.27%	1.03%
Risk-free interest rate maximum	2.44%	2.39%	2.63%	2.39%
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 7 days	8 months 15 days	8 months 8 days	8 months 15 days
Weighted-average volatility	60.00%	61.00%	60.00%	61.00%
Risk-free interest rate minimum	2.36%	1.20%	2.36%	1.20%
Risk-free interest rate maximum	2.59%	1.93%	2.59%	1.93%
Expected dividends	0.00%	0.00%	0.00%	0.00%
Market-based restricted stock units
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average volatility					66.00%
Risk-free interest rate					2.79%
Expected dividends					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

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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

Rate of weighted-average expected volatility for award under share-based payment arrangement.

+ 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.

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- Definition

Expected term of award under share-based payment arrangement, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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=116856206&loc=d3e5070-113901

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-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=115993241&loc=d3e301413-122809

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