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Average number of employees - Summary of average number of employees (Detail) - Employee
 12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Disclosure of average number of employees [Line Items]      
Average number of employees 47,458 46,807 51,029
Investment in equity accounted units [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 3,129 3,108 3,507
Australia and New Zealand [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 19,595 19,643 21,338
Australia and New Zealand [member] | Investment in equity accounted units [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 578 602 849
Canada [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 10,620 10,256 10,239
United Kingdom [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 287 309 487
Europe [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,418 1,505 1,722
Africa [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 4,758 4,730 6,145
Africa [Member] | Investment in equity accounted units [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,262 1,269 1,270
United States [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 3,792 3,429 3,196
Mongolia [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 2,886 2,861 2,737
Indonesia [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,615 1,642 2,862
South America [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,499 1,434 1,554
South America [Member] | Investment in equity accounted units [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,289 1,237 1,388
Other countries [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 988 998 749
Subsidiaries And Joint Operations [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 44,329 43,699 47,522
Subsidiaries And Joint Operations [Member] | Australia and New Zealand [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 19,017 19,041 20,489
Subsidiaries And Joint Operations [Member] | Canada [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 10,620 10,256 10,239
Subsidiaries And Joint Operations [Member] | United Kingdom [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 287 309 487
Subsidiaries And Joint Operations [Member] | Europe [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,418 1,505 1,722
Subsidiaries And Joint Operations [Member] | Africa [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 3,496 3,461 4,875
Subsidiaries And Joint Operations [Member] | United States [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 3,792 3,429 3,196
Subsidiaries And Joint Operations [Member] | Mongolia [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 2,886 2,861 2,737
Subsidiaries And Joint Operations [Member] | Indonesia [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,615 1,642 2,862
Subsidiaries And Joint Operations [Member] | South America [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 210 197 166
Subsidiaries And Joint Operations [Member] | Other countries [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 988 998 749