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Significant accounting policies (Tables)

12 Months Ended

Dec. 31, 2017

The estimated useful lives for the current period and comparative period are as follows:

	Useful life (years)		Average annual depreciation rate
Machinery and equipment		10	10%
Office furniture and equipment		10	10%
Computer equipment	3.3 – 4		30% - 25%
Transportation equipment	4 – 5		25% - 20%
Communication equipment	10 – 4 – 3.3		10% - 25% - 30%
Improvements on leased buildings		10	10%

Amortization periods for the current and comparative period are as follows:

	Period (years)	Average annual amortization rate
Improvements to concession assets	12.5 – 20	8% - 5%
Airport concessions	18 – 49	5.5% - 2%
Rights to use airport facilities	10 – 49	10% - 2%
Other acquired rights	44 – 48	2%