Stock-based compensation - Summary of incentive plan for members of the Board of Directors (Details) - Directors - $ / shares	3 Months Ended	9 Months Ended	12 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Restricted Stock Award
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward
Non-vested at beginning of period			0	0	0
Granted	0	0	0	1,052	1,052
Vested			0		(1,052)
Forfeited			0		0
Non-vested at end of period	0		0		0
Weighted-Average Grant Date Fair Value
Non-vested at beginning of period			$ 0	$ 0	$ 0
Granted			0		49.25
Vested			0		49.25
Forfeited			0		0
Non-vested at end of period	$ 0		$ 0		$ 0
Restricted Stock Units (RSUs)
Share Based Compensation Arrangement By Share Based Payment Award Equity Instruments Other Than Options Nonvested Roll Forward
Non-vested at beginning of period			0	0	0
Granted	783	1,989	43,084	27,449	27,449
Vested			(43,084)		(27,449)
Forfeited			0		0
Non-vested at end of period	0		0		0
Weighted-Average Grant Date Fair Value
Non-vested at beginning of period			$ 0	$ 0	$ 0
Granted			35.46		57.64
Vested			35.46		57.64
Forfeited			0		0
Non-vested at end of period	$ 0		$ 0		$ 0

No definition available.

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The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period.

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Weighted average fair value as of the grant date of equity-based award plans other than stock (unit) option plans that were not exercised or put into effect as a result of the occurrence of a terminating event.

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The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The weighted average fair value at grant date for nonvested equity-based awards issued during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan).

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The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

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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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No definition available.

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Per share or unit weighted-average fair value of nonvested award under share-based payment arrangement. Excludes share and unit options.

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The number of equity-based payment instruments, excluding stock (or unit) options, that vested during the reporting period.

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The weighted average fair value as of grant date pertaining to an equity-based award plan other than a stock (or unit) option plan for which the grantee gained the right during the reporting period, by satisfying service and performance requirements, to receive or retain shares or units, other instruments, or cash in accordance with the terms of the arrangement.

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