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Fair value measurements (Details) - USD ($) $ in Thousands | Dec. 31, 2017 | Dec. 31, 2016
--- | --- | ---
ASSETS: |  |
Derivative instruments | $ 1,724 |
LIABILITIES: |  |
Derivative instruments | 1,200 |
Recurring [Member] |  |
ASSETS: |  |
Cash equivalents | 142,819 | $ 79,014
Trust assets | 1,757 | 3,147
Surety bonds | 1,618 | 1,874
Derivative instruments | 1,724 | 925
TOTAL | 147,918 | 84,960
LIABILITIES: |  |
Contingent obligation | 0 | 0
Derivative instruments | 1,200 | 253
TOTAL | 1,200 | 253
Recurring [Member] | Level 1 [Member] |  |
ASSETS: |  |
Cash equivalents | 142,819 | 79,014
Trust assets | 1,757 | 3,147
Surety bonds | 1,618 | 1,874
Derivative instruments | 0 | 0
TOTAL | 146,194 | 84,035
LIABILITIES: |  |
Contingent obligation | 0 | 0
Derivative instruments | 0 | 0
TOTAL | 0 | 0
Recurring [Member] | Level 2 [Member] |  |
ASSETS: |  |
Cash equivalents | 0 | 0
Trust assets | 0 | 0
Surety bonds |  | 0
Derivative instruments | 0 | 0
TOTAL | 0 | 0
LIABILITIES: |  |
Contingent obligation | 0 | 0
Derivative instruments | 0 | 0
TOTAL | 0 | 0
Recurring [Member] | Level 3 [Member] |  |
ASSETS: |  |
Cash equivalents | 0 | 0
Trust assets | 0 | 0
Surety bonds | 0 | 0
Derivative instruments | 1,724 | 925
TOTAL | 1,724 | 925
LIABILITIES: |  |
Contingent obligation | 0 | 0
Derivative instruments | 1,200 | 253
TOTAL | $ 1,200 | $ 253

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