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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

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