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Fair value measurements (Details) - Recurring [Member] - USD ($) $ in Thousands | Dec. 31, 2015 | Dec. 31, 2014
--- | --- | ---
ASSETS: |  |
Cash equivalents | $ 102,675 | $ 137,712
Trust assets | 5,226 | 8,067
Surety bonds | 1,870 | 1,987
TOTAL | 109,771 | 147,766
LIABILITIES: |  |
E.ON contingent obligation – net | 0 | 0
TOTAL | 0 | 0
Level 1 [Member] |  |
ASSETS: |  |
Cash equivalents | 102,675 | 137,712
Trust assets | 5,226 | 8,067
Surety bonds | 1,870 | 1,987
TOTAL | 109,771 | 147,766
LIABILITIES: |  |
E.ON contingent obligation – net | 0 | 0
TOTAL | 0 | 0
Level 2 [Member] |  |
ASSETS: |  |
Cash equivalents | 0 | 0
Trust assets | 0 | 0
Surety bonds | 0 | 0
TOTAL | 0 | 0
LIABILITIES: |  |
E.ON contingent obligation – net | 0 | 0
TOTAL | 0 | 0
Level 3 [Member] |  |
ASSETS: |  |
Cash equivalents | 0 | 0
Trust assets | 0 | 0
Surety bonds | 0 | 0
TOTAL | 0 | 0
LIABILITIES: |  |
E.ON contingent obligation – net | 0 | 0
TOTAL | $ 0 | $ 0

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- Definition Fair values as of the balance sheet date of all surety bonds for workers compensation insurance as of the balance sheet date. + References No definition available. + Details
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- Definition Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=36462937&loc=d3e19207-110258 + Details
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