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Share-based compensation - Dividend equivalent rights - Summary of awards (Details) | Mar. 04, 2021 | Mar. 04, 2020 | Mar. 07, 2019 | Apr. 04, 2018 | Jan. 15, 2016 | Mar. 06, 2015 | Sep. 01, 2014 | Mar. 12, 2014
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Share-based compensation. |  |  |  |  |  |  |  |
Valuation Method | Black-Scholes | Black-Scholes | Black-Scholes | Black-Scholes | Monte Carlo | Monte Carlo | Monte Carlo | Monte Carlo

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- Definition Description of method used to estimate fair value of award under share-based payment arrangement. + References 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 (f)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
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