Shareholders' equity (Schedule of Fair Value of Options Estimated on Grant Date) (Details) - $ / shares	3 Months Ended	12 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility	46.00%	50.00%	50.00%
Expected term in years					6 years 3 months
Risk-free interest rate	1.40%	2.59%
Estimated fair value of underlying ordinary shares	$ 27.9
Dividend yield	0.00%	0.00%	0.00%	0.00%	0.00%
Minimum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility				45.00%	51.00%
Expected term in years	4 years 5 months 1 day	5 years 6 months 22 days	5 years	5 years 3 months
Risk-free interest rate			1.52%	2.00%	2.00%
Estimated fair value of underlying ordinary shares		$ 20.0	$ 12.8	$ 8.69	$ 4.75
Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Volatility				50.00%	56.00%
Expected term in years	5 years 6 months 22 days	6 years 1 month 10 days	6 years 1 month 10 days	6 years 3 months
Risk-free interest rate			2.59%	3.07%	2.40%
Estimated fair value of underlying ordinary shares		$ 23.08	$ 22.64	$ 18.47	$ 5.56

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 estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

+ References

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- Definition

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

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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

Price of a single share of a number of saleable stocks of a company.

+ 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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