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Financial - risk management objectives and policies (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Disclosure of financial risk management objectives and policies [Line Items] |  |  |
Increase in exchange rate basis points | 10.00% | 10.00% | 10.00%
Decrease in exchange rate basis points | (10.00%) | (10.00%) | (10.00%)
Exchange rate increased effect on income | $ 2,474 | $ (924) | $ 6,233
Exchange rate decreased effect on income | $ (2,459) | $ 926 | $ (7,618)

X
- Definition The percentage of decrease in exchange rate basis points. + References No definition available. + Details
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Namespace Prefix: | cik0001013131_
Data Type: | num:percentItemType
Balance Type: | na
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X
- References No definition available. + Details
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Namespace Prefix: | cik0001013131_
Data Type: | xbrli:stringItemType
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X
- Definition The amount of decrease in income due to decrease the basis points in exchange rate. + References No definition available. + Details
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Namespace Prefix: | cik0001013131_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

X
- Definition The amount of increase in income due to Increase the basis points in exchange rate. + References No definition available. + Details
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Namespace Prefix: | cik0001013131_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
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X
- Definition The percentage of increase in exchange rate basis points. + References No definition available. + Details
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Namespace Prefix: | cik0001013131_
Data Type: | num:percentItemType
Balance Type: | na
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