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Operating leases - Non-cancellable Operating Lease Rentals (Details)
 12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Disclosure of maturity analysis of operating lease payments [line items]    
Minimum lease payments payable under non-cancellable operating lease 7600000 181600000
Less than one year    
Disclosure of maturity analysis of operating lease payments [line items]    
Minimum lease payments payable under non-cancellable operating lease 2100000 22200000
Between one and three years    
Disclosure of maturity analysis of operating lease payments [line items]    
Minimum lease payments payable under non-cancellable operating lease 3600000 38500000
Between three and five years    
Disclosure of maturity analysis of operating lease payments [line items]    
Minimum lease payments payable under non-cancellable operating lease 1900000 26000000
More than five years    
Disclosure of maturity analysis of operating lease payments [line items]    
Minimum lease payments payable under non-cancellable operating lease 0 94900000