Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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Number of options outstanding, including both vested and non-vested options.

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Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Stock options - Plans (Details) - shares	1 Months Ended		9 Months Ended
Stock options - Plans (Details) - shares	Jan. 31, 2019	Jun. 30, 2017	Sep. 30, 2019	Dec. 31, 2018
Stock options
Number of shares outstanding			4,718,597	3,746,567
2007 Plan
Stock options
Number of shares outstanding		3,141,625
2017 Plan
Stock options
Number of shares authorized		2,255,000
Cumulative increase in number of shares issuable (as a percent)		4.00%
Additional shares authorized	929,378
Number of shares available for future issuance			1,983,432
Vesting period (in years)			4 years
2017 Plan \| Maximum
Stock options
Expiration period			10 years