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Costs (Details 3) - Number
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Subsidiaries [Member]      
Disclosure Of Operating Expenses [Line Items]      
Average number of employees 31,573 32,464 33,061
Subsidiaries [Member] | Senior Managers [Member]      
Disclosure Of Operating Expenses [Line Items]      
Average number of employees 999 995 1,018
Subsidiaries [Member] | Junior Managers [Member]      
Disclosure Of Operating Expenses [Line Items]      
Average number of employees 9,095 9,089 9,160
Subsidiaries [Member] | Employees [Member]      
Disclosure Of Operating Expenses [Line Items]      
Average number of employees 16,220 16,721 17,180
Subsidiaries [Member] | Workers [Member]      
Disclosure Of Operating Expenses [Line Items]      
Average number of employees 5,259 5,659 5,703
Joint operations [member]      
Disclosure Of Operating Expenses [Line Items]      
Average number of employees 745 771 805
Joint operations [member] | Senior Managers [Member]      
Disclosure Of Operating Expenses [Line Items]      
Average number of employees 17 17 18
Joint operations [member] | Junior Managers [Member]      
Disclosure Of Operating Expenses [Line Items]      
Average number of employees 84 98 109
Joint operations [member] | Employees [Member]      
Disclosure Of Operating Expenses [Line Items]      
Average number of employees 361 371 384
Joint operations [member] | Workers [Member]      
Disclosure Of Operating Expenses [Line Items]      
Average number of employees 283 285 294