Stock incentive plans - Stock option awards (Details) - Stock options - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2019	Dec. 31, 2018
Stock incentive plans
Vesting period	4 years
Number of Options
Outstanding at beginning of period (in shares)	304
Exercised (in shares)	(27)
Canceled/forfeited (in shares)	0
Outstanding and exercisable at end of period (in shares)	277	304
Weighted Average Exercise Price
Outstanding at beginning of period (in dollars per share)	$ 7.49
Exercised (in dollars per share)	5.72
Canceled/forfeited (in dollars per share)	0
Outstanding and exercisable at end of period (in dollars per share)	$ 7.66	$ 7.49
Weighted-Average Remaining Contract Life (years)
Outstanding and exercisable at end of period	3 years 1 month 6 days	3 years 9 months 18 days
Aggregate Intrinsic Value
Outstanding and exercisable at end of period	$ 1,044	$ 3,970
Vesting in 12 months
Stock incentive plans
Vesting period	12 months
Vesting percentage	25.00%
Vesting monthly 12 months after grant date
Stock incentive plans
Vesting percentage	75.00%

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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The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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

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A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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Percentage of vesting of award under share-based payment arrangement.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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