Share-based awards - Summary of RSU and PSU Activity (Details) - $ / shares	6 Months Ended	12 Months Ended
Jun. 30, 2020	Dec. 31, 2019
Performance Share Unit (PSUs)
Outstanding Number of Shares
Outstanding at beginning of period (in shares)	175,989
Granted (in shares)	57,184
Shares vested (in shares)	(63,516)
Forfeited (in shares)	(197)
Outstanding at end of period (in shares)	169,460	175,989
Weighted Average Grant Date Fair Value
Outstanding at beginning of period (USD per share)	$ 110.79
Granted (USD per share)	165.30
Shares vested (USD per share)	83.92
Forfeited (USD per share)	83.47
Outstanding at end of period (USD per share)	$ 137.53	$ 110.79
Weighted Average Remaining Contractual Life
Outstanding at end of period (in years)	1 year 7 months 20 days	1 year 14 days
Restricted Stock Units (RSUs)
Outstanding Number of Shares
Outstanding at beginning of period (in shares)	389,900
Granted (in shares)	125,756
Shares vested (in shares)	(138,939)
Forfeited (in shares)	(16,994)
Outstanding at end of period (in shares)	359,723	389,900
Weighted Average Grant Date Fair Value
Outstanding at beginning of period (USD per share)	$ 119.07
Granted (USD per share)	165.18
Shares vested (USD per share)	98.20
Forfeited (USD per share)	126.73
Outstanding at end of period (USD per share)	$ 142.86	$ 119.07
Weighted Average Remaining Contractual Life
Outstanding at end of period (in years)	1 year 9 months 25 days	1 year 5 months 4 days

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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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- 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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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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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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Weighted average remaining contractual term for equity-based awards excluding options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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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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