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Bank borrowings - Amended Revolving Credit Facility - (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Borrowings [abstract] |  |  |
Borrowings | $ 141,012 | $ 1,003 |
Repaid | $ 106,514 | $ 453,948 | $ 81,237
Minimum |  |  |
Borrowings [abstract] |  |  |
Applicable margin (as a percent) | 0.00% |  |
New Revolving Credit Facility |  |  |
Borrowings [abstract] |  |  |
Interest rate (as a percent) | 5.20% |  |
LIBOR |  |  |
Borrowings [abstract] |  |  |
Applicable margin (as a percent) | 1.00% |  |
Base rate | New Revolving Credit Facility |  |  |
Borrowings [abstract] |  |  |
Applicable margin (as a percent) | 2.25% |  |
Eurocurrency Rate Basis | New Revolving Credit Facility |  |  |
Borrowings [abstract] |  |  |
Applicable margin (as a percent) | 3.25% |  |
Fed Overnight Bank Funding Rate |  |  |
Borrowings [abstract] |  |  |
Applicable margin (as a percent) | 0.50% |  |

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- Definition The amount of outstanding funds that the entity is obligated to repay. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IAS -Number 1 -IssueDate 2018-01-01 -Paragraph 55 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=1&date=2018-03-01&anchor=para_55&doctype=Standard -URIDate 2018-03-16 + Details
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- Definition The adjustment to the basis (reference rate) used for calculation of the interest rate on borrowings. [Refer: Borrowings] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IFRS -Number 7 -IssueDate 2018-01-01 -Paragraph 7 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2018-03-01&anchor=para_7&doctype=Standard -URIDate 2018-03-16 + Details
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- Definition The interest rate on borrowings. [Refer: Borrowings] + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Name IFRS -Number 7 -IssueDate 2018-01-01 -Paragraph 7 -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IFRS&num=7&date=2018-03-01&anchor=para_7&doctype=Standard -URIDate 2018-03-16 + Details
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- Definition The cash outflow to settle borrowings, classified as financing activities. [Refer: Borrowings] + References Reference 1: http://www.xbrl.org/2003/role/exampleRef -Name IAS -Number 7 -IssueDate 2018-01-01 -Paragraph 17 -Subparagraph d -URI http://eifrs.ifrs.org/eifrs/XBRL?type=IAS&num=7&date=2018-03-01&anchor=para_17_d&doctype=Standard -URIDate 2018-03-16 + Details
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