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Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands | Mar. 31, 2018 | Dec. 31, 2017
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Detail of regulatory liabilities |  |
Regulatory liabilities, current | $ 136,535 | $ 100,086
Regulatory liabilities, non-current | 2,415,417 | 2,452,536
Excess deferred income taxes - Tax Cuts and Jobs Act |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 0 | 0
Regulatory liabilities, non-current | 1,519,224 | 1,520,274
Asset retirement obligations |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 0 | 0
Regulatory liabilities, non-current | 315,922 | 332,171
Removal costs |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 26,949 | 18,238
Regulatory liabilities, non-current | 197,274 | 209,191
Other postretirement benefits |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 37,642 | 37,642
Regulatory liabilities, non-current | 142,560 | 151,985
Income taxes — deferred investment tax credit |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 2,144 | 2,164
Regulatory liabilities, non-current | 52,478 | 52,497
Income taxes — change in rates |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 2,799 | 2,573
Regulatory liabilities, non-current | 73,703 | 70,537
Spent nuclear fuel |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 6,609 | 6,924
Regulatory liabilities, non-current | 61,736 | 62,132
Renewable energy standard (c) |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 32,694 | 23,155
Regulatory liabilities, non-current | 0 | 0
Demand side management (c) |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 4,049 | 3,066
Regulatory liabilities, non-current | 4,123 | 4,921
Sundance maintenance |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 0 | 0
Regulatory liabilities, non-current | 17,299 | 16,897
Deferred gains on utility property |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 4,423 | 4,423
Regulatory liabilities, non-current | 9,873 | 10,988
Four Corners coal reclamation |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 1,858 | 1,858
Regulatory liabilities, non-current | 18,525 | 18,921
Tax expense adjustor mechanism |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 15,676 | 0
Regulatory liabilities, non-current | 0 | 0
Other |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 1,692 | 43
Regulatory liabilities, non-current | $ 2,700 | $ 2,022

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- Definition The amount for the individual regulatory current liability including deferred fuel and purchased power as itemized in a table of regulatory current liabilities as of the end of the period. + References No definition available. + Details
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- Definition The amount for the individual regulatory noncurrent liability as itemized in a table of regulatory noncurrent liabilities as of the end of the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.24) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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