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Share-based awards - Summary of Movement in Non-Vested Share Options (Details) - Employee Stock Option - $ / shares	6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020
Options Outstanding Number of Shares
Non-vested outstanding at beginning of period (in shares)	328,901
Granted (in shares)	100,299
Vested (in shares)	(116,117)
Forfeited (in shares)	0
Non-vested outstanding at end of period (in shares)	313,083
Weighted Average Exercise Price
Non-vested outstanding at beginning of period (USD per share)	$ 125.89
Granted (USD per share)	177.76
Vested (USD per share)	109.53
Forfeited (USD per share)	0
Non-vested outstanding at end of period (USD per share)	148.58
Weighted Average Grant Date Fair Value
Non-vested outstanding at beginning of period (USD per share)	36.24
Granted (USD per share)	49.15	$ 42.43
Vested (USD per share)	31.84
Forfeited (USD per share)	0
Non-vested outstanding at end of period (USD per share)	$ 42.00

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Share Based Compensation Arrangement by Share Based Payment Award, Nonvested Options, Forfeited in Period Weighted Average Exercise Price

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Gross number of share options (or share units) granted during the period.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901

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