| 2017 | 2016 | 2015 | ||||||||||
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Net book value of property, plant and equipment |
382,630 | 345,679 | 274,122 | |||||||||
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Provision for obsolescence of materials and equipment |
(1,652 | ) | (1,380 | ) | (762 | ) | ||||||
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Provision for impairment of property, plant and equipment |
(26,535 | ) | (36,285 | ) | (2,455 | ) | ||||||
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| 354,443 | 308,014 | 270,905 | ||||||||||
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