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Directors and Key Management compensation - Additional Information (Details)
12 Months Ended
Dec. 31, 2019
executive_officer
director
Dec. 31, 2018
executive_officer
director
Dec. 31, 2017
executive_officer
director
Related Party [Abstract]      
Number of directors accruing benefits under money purchase schemes | director 2 3 2
Number of executive Officers accruing benefits under share-based payment schemes | executive_officer 2 3 2