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Fair value measurements (Details) - Fair Value, Measurements, Recurring [Member] - USD ($) $ in Thousands | Mar. 31, 2016 | Dec. 31, 2015
--- | --- | ---
ASSETS: |  |
Cash equivalents | $ 120,061 | $ 102,675
Trust assets | 4,789 | 5,226
Surety bonds | 1,870 | 1,870
TOTAL | 126,720 | 109,771
LIABILITIES: |  |
E.ON contingent obligation - net | 0 | 0
TOTAL | 0 | 0
Fair Value, Inputs, Level 1 [Member] |  |
ASSETS: |  |
Cash equivalents | 120,061 | 102,675
Trust assets | 4,789 | 5,226
Surety bonds | 1,870 | 1,870
TOTAL | 126,720 | 109,771
LIABILITIES: |  |
E.ON contingent obligation - net | 0 | 0
TOTAL | 0 | 0
Fair Value, Inputs, 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
Fair Value, Inputs, 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 as of the balance sheet date of all trust assets 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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- Definition Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash -URI http://asc.fasb.org/extlink&oid=6506951 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Cash Equivalents -URI http://asc.fasb.org/extlink&oid=6507016 + Details
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