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Average number of employees - Summary of average number of employees (Detail) - Employee
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of average number of employees [Line Items]      
Average number of employees 46,807 51,029 54,938
Investment in equity accounted units [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 3,108 3,507 3,451
Australia and New Zealand [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 19,643 21,338 22,988
Australia and New Zealand [member] | Investment in equity accounted units [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 602 849 863
Canada [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 10,256 10,239 11,113
United Kingdom [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 309 487 542
Europe [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,505 1,722 2,312
Africa [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 4,730 6,145 6,922
Africa [Member] | Investment in equity accounted units [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,269 1,270 1,271
United States [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 3,429 3,196 3,439
Mongolia [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 2,861 2,737 2,647
Indonesia [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,642 2,862 2,871
South America [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,434 1,554 1,509
South America [Member] | Investment in equity accounted units [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,237 1,388 1,317
Other countries [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 998 749 595
Subsidiaries and joint operations [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 43,699 47,522 51,487
Subsidiaries and joint operations [member] | Australia and New Zealand [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 19,041 20,489 22,125
Subsidiaries and joint operations [member] | Canada [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 10,256 10,239 11,113
Subsidiaries and joint operations [member] | United Kingdom [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 309 487 542
Subsidiaries and joint operations [member] | Europe [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,505 1,722 2,312
Subsidiaries and joint operations [member] | Africa [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 3,461 4,875 5,651
Subsidiaries and joint operations [member] | United States [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 3,429 3,196 3,439
Subsidiaries and joint operations [member] | Mongolia [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 2,861 2,737 2,647
Subsidiaries and joint operations [member] | Indonesia [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 1,642 2,862 2,871
Subsidiaries and joint operations [member] | South America [Member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 197 166 192
Subsidiaries and joint operations [member] | Other countries [member]      
Disclosure of average number of employees [Line Items]      
Average number of employees 998 749 595