Stock-based compensation - Valuation assumptions of stock-based awards (Details)	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 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
Weighted-average volatility	0.00%	65.00%
Risk-free interest rate	0.00%	2.27%
Expected dividends	0.00%	0.00%
Stock Appreciation Units (SAUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average expected term (years)	1 year 8 months 1 day	2 years 1 month 6 days
Weighted-average volatility	64.00%	67.00%
Risk-free interest rate minimum	2.45%	1.03%
Risk-free interest rate maximum	2.63%	1.98%
Expected dividends	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 22 days	8 months 22 days
Weighted-average volatility	61.00%	61.00%
Risk-free interest rate minimum	2.36%	1.20%
Risk-free interest rate maximum	2.59%	1.31%
Expected dividends	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

+ 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.

+ 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 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://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=109197908&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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