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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, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
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	2,765	1,052		25,159
Vested			(1,052)		(25,159)
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			49.25		46.71
Vested			49.25		46.71
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	1,989		27,449	25,159	0
Vested			(27,449)		0
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			57.64		0
Vested			57.64		0
Forfeited			0		0
Non-vested at end of period	$ 0		$ 0		$ 0

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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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The weighted average fair value of nonvested awards on equity-based plans excluding option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, revenue or profit achievement stock award plan) for which the employer is contingently obligated to issue equity instruments or transfer assets to an employee who has not yet satisfied service or performance criteria necessary to gain title to proceeds from the sale of the award or underlying shares or units.

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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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