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Regulatory Matters - Schedule of Regulatory Liabilities (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
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Detail of regulatory liabilities |  |
Regulatory liabilities, current | $ 156,757 | $ 100,086
Regulatory liabilities, non-current | 2,389,002 | 2,452,536
Asset retirement obligations |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 0 | 0
Regulatory liabilities, non-current | 319,793 | 332,171
Removal costs |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 28,879 | 18,238
Regulatory liabilities, non-current | 191,104 | 209,191
Other postretirement benefits |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 37,842 | 37,642
Regulatory liabilities, non-current | 133,109 | 151,985
Income taxes — deferred investment tax credit |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 2,137 | 2,164
Regulatory liabilities, non-current | 51,784 | 52,497
Income taxes — change in rates |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 2,799 | 2,573
Regulatory liabilities, non-current | 72,790 | 70,537
Spent nuclear fuel |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 6,617 | 6,924
Regulatory liabilities, non-current | 59,873 | 62,132
Renewable energy standard |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 38,986 | 23,155
Regulatory liabilities, non-current | 0 | 0
Demand side management |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 10,187 | 3,066
Regulatory liabilities, non-current | 4,124 | 4,921
Sundance maintenance |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 0 | 0
Regulatory liabilities, non-current | 17,701 | 16,897
Deferred gains on utility property |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 4,423 | 4,423
Regulatory liabilities, non-current | 8,790 | 10,988
Four Corners coal reclamation |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 1,858 | 1,858
Regulatory liabilities, non-current | 18,296 | 18,921
Tax expense adjustor mechanism |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 13,865 | 0
Regulatory liabilities, non-current | 0 | 0
Other |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 2,918 | 43
Regulatory liabilities, non-current | 815 | 2,022
ACC | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 0 | 0
Regulatory liabilities, non-current | 1,265,229 | 1,266,104
United States Federal Energy Regulatory Commission | Excess deferred income taxes - ACC - Tax Cuts and Jobs Act |  |
Detail of regulatory liabilities |  |
Regulatory liabilities, current | 6,246 | 0
Regulatory liabilities, non-current | $ 245,594 | $ 254,170

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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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