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Average number of employees - Summary of average number of employees (Detail) - Employee | 12 Months Ended
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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

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