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Fair value measurements (Details) - USD ($) $ in Thousands | Mar. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Jun. 30, 2015
--- | --- | --- | --- | ---
ASSETS: |  |  |  |
Derivative instruments | $ 1,610 |  |  |
Recurring [Member] |  |  |  |
ASSETS: |  |  |  |
Cash equivalents | 103,286 | $ 79,014 | $ 102,675 |
Trust assets | 3,138 | 3,147 | 5,226 |
Surety bonds | 1,618 | 1,874 | 1,870 |
Derivative instruments | 1,610 | 925 |  |
TOTAL | 109,652 | 84,960 | 109,771 |
LIABILITIES: |  |  |  |
E.ON contingent obligation – net | 0 | 0 | 0 |
Derivative instruments | 14,915 | 253 |  |
TOTAL | 14,915 | 253 | 0 |
Recurring [Member] | Level 1 [Member] |  |  |  |
ASSETS: |  |  |  |
Cash equivalents | 103,286 | 79,014 | 102,675 |
Trust assets | 3,138 | 3,147 | 5,226 |
Surety bonds | 1,618 | 1,874 | 1,870 |
Derivative instruments | 0 | 0 |  |
TOTAL | 108,042 | 84,035 | 109,771 |
LIABILITIES: |  |  |  |
E.ON contingent obligation – net | 0 | 0 |  | $ 0
Derivative instruments | 0 | 0 |  |
TOTAL | 0 | 0 | 0 |
Recurring [Member] | Level 2 [Member] |  |  |  |
ASSETS: |  |  |  |
Cash equivalents | 0 | 0 | 0 |
Trust assets | 0 | 0 | 0 |
Surety bonds | 0 | 0 | 0 |
Derivative instruments | 0 | 0 |  |
TOTAL | 0 | 0 | 0 |
LIABILITIES: |  |  |  |
E.ON contingent obligation – net | 0 | 0 | 0 |
Derivative instruments | 13,015 | 0 |  |
TOTAL | 13,015 | 0 | 0 |
Recurring [Member] | Level 3 [Member] |  |  |  |
ASSETS: |  |  |  |
Cash equivalents | 0 | 0 | 0 |
Trust assets | 0 | 0 | 0 |
Surety bonds | 0 | 0 | 0 |
Derivative instruments | 1,610 | 925 |  |
TOTAL | 1,610 | 925 | 0 |
LIABILITIES: |  |  |  |
E.ON contingent obligation – net | 0 | 0 | 0 |
Derivative instruments | 1,900 | 253 |  |
TOTAL | $ 1,900 | $ 253 | $ 0 |

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