Stock-based compensation - Summary of incentive plan for members of the Board of Directors (Details) - Directors - $ / shares	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
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	1,052	25,159	25,771
Vested	(1,052)	(25,159)	(25,771)
Forfeited	0	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	49.25	46.71	38.42
Vested	49.25	46.71	38.42
Forfeited	0	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	27,449	0	0
Vested	(27,449)	0	0
Forfeited	0	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	57.64	0	0
Vested	57.64	0	0
Forfeited	0	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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-Publisher FASB
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- Definition

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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- Definition

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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- Definition

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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- Definition

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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-Publisher FASB
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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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