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Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013
Regulatory Assets [Line Items] |  |  |
Regulatory Assets | $ 157,737 | $ 165,183 |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities | 30,300 | 22,715 |
Debt make-whole premium | 0 | 0 | $ 6,757
Conservation Costs Regulatory Liability [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities | 231 | 680 |
Deferred Fuel Savings [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities | 9,515 | 755 |
Elm Road Regulatory Liability [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities | 643 | 1,497 |
Income Taxes [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities | 1,559 | 1,794 |
Non-ARO Removal Cost [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities | 17,137 | 16,129 |
Renewable Energy Credits [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities | 327 | 753 |
Other Regulatory Liability [Member] |  |  |
Regulatory Liabilities [Line Items] |  |  |
Regulatory Liabilities | 888 | 1,107 |
Asset Retirement Obligation [Member] |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets | 4,849 | 4,532 |
Debt Related Costs [Member] |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets | 10,672 | 11,133 |
Derivatives [Member] |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets | 54,083 | 54,998 |
Environmental Costs [Member] |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets | 368 | 700 |
Tax Recovery AFUDC Equity [Member] |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets | 8,950 | 8,821 |
Unfunded Pension and Other Postretirement Plans Costs [Member] |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets | 78,181 | 84,551 |
Other Regulatory Asset [Member] |  |  |
Regulatory Assets [Line Items] |  |  |
Regulatory Assets | $ 634 | $ 448 |

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- Definition The amount for the individual regulatory asset as itemized in a table of regulatory assets as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6499975&loc=d3e44250-110382 + Details
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