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Costs - Average number of employees (Details) - employee
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Subsidiaries      
Costs      
Average number of employees 31,048 31,136 31,573
Subsidiaries | Senior Managers      
Costs      
Average number of employees 993 1,014 999
Subsidiaries | Junior Managers      
Costs      
Average number of employees 9,280 9,267 9,095
Subsidiaries | Employees      
Costs      
Average number of employees 15,995 15,945 16,220
Subsidiaries | Workers      
Costs      
Average number of employees 4,780 4,910 5,259
Joint operations      
Costs      
Average number of employees 726 741 745
Joint operations | Senior Managers      
Costs      
Average number of employees 17 16 17
Joint operations | Junior Managers      
Costs      
Average number of employees 73 77 84
Joint operations | Employees      
Costs      
Average number of employees 349 361 361
Joint operations | Workers      
Costs      
Average number of employees 287 287 283