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Balance Sheet Components Balance Sheet Components (Notes) | 9 Months Ended
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Sep. 30, 2018
Balance Sheet Components [Abstract] |
Supplemental Balance Sheet Disclosures [Text Block] | BALANCE SHEET COMPONENTS Materials and Supplies, Fuel Stock and Stored Natural Gas Inventories of materials and supplies, fuel stock and stored natural gas are recorded at average cost for our regulated operations and the lower of cost or market for our non-regulated operations and consisted of the following as of September 30, 2018 and December 31, 2017 (dollars in thousands):
| September 30, |  | December 31,
| 2018 |  | 2017
Materials and supplies | $ | 45,169 |  |  | $ | 41,493 |
Fuel stock | 5,347 |  |  | 4,843 |
Stored natural gas | 12,250 |  |  | 11,739 |
Total | $ | 62,766 |  |  | $ | 58,075 |
Net Utility Property Net utility property consisted of the following as of September 30, 2018 and December 31, 2017 (dollars in thousands):
| September 30, |  | December 31,
| 2018 |  | 2017
Utility plant in service | $ | 6,060,373 |  |  | $ | 5,853,308 |
Construction work in progress | 180,237 |  |  | 157,839 |
Total | 6,240,610 |  |  | 6,011,147 |
Less: Accumulated depreciation and amortization | 1,685,170 |  |  | 1,612,337 |
Total net utility property | $ | 4,555,440 |  |  | $ | 4,398,810 |
Other Current Liabilities Other current liabilities consisted of the following as of September 30, 2018 and December 31, 2017 (dollars in thousands):
| September 30, |  | December 31,
| 2018 |  | 2017
Accrued taxes other than income taxes | $ | 37,723 |  |  | $ | 33,802 |
Current unsettled interest rate swap derivative liabilities | — |  |  | 34,447 |
Employee paid time off accruals | 20,644 |  |  | 20,330 |
Accrued interest | 30,343 |  |  | 16,351 |
Current portion of pensions and other postretirement benefits | 10,036 |  |  | 11,544 |
Utility energy commodity derivative liabilities | 6,853 |  |  | 8,848 |
Other current liabilities | 27,805 |  |  | 33,791 |
Total other current liabilities | $ | 133,404 |  |  | $ | 159,113 |
Regulatory Assets and Liabilities Regulatory assets and liabilities consisted of the following as of September 30, 2018 and December 31, 2017 (dollars in thousands):
| September 30, 2018 |  | December 31, 2017
| Current |  | Non-Current |  | Current |  | Non-Current
Regulatory Assets |  |  |  |  |  |  |
Energy commodity derivatives | $ | 19,238 |  |  | $ | 9,555 |  |  | $ | 24,991 |  |  | $ | 18,967 |
Decoupling surcharge | 2,287 |  |  | 18,099 |  |  | 19,759 |  |  | 2,600 |
Pension and other postretirement benefit plans | — |  |  | 201,620 |  |  | — |  |  | 209,115 |
Interest rate swaps | — |  |  | 131,972 |  |  | — |  |  | 169,704 |
Deferred income taxes | — |  |  | 90,285 |  |  | — |  |  | 90,315 |
Settlement with Coeur d'Alene Tribe | — |  |  | 42,971 |  |  | — |  |  | 43,954 |
Demand side management programs | — |  |  | 19,774 |  |  | — |  |  | 24,620 |
Utility plant to be abandoned | — |  |  | 24,158 |  |  | — |  |  | 24,330 |
Other regulatory assets | — |  |  | 38,429 |  |  | — |  |  | 35,794 |
Total regulatory assets | $ | 21,525 |  |  | $ | 576,863 |  |  | $ | 44,750 |  |  | $ | 619,399 |
| September 30, 2018 |  | December 31, 2017
| Current |  | Non-Current |  | Current |  | Non-Current
Regulatory Liabilities |  |  |  |  |  |  |
Income tax related liabilities | $ | 36,850 |  |  | $ | 433,714 |  |  | $ | — |  |  | $ | 460,542 |
Deferred natural gas costs | 35,442 |  |  | — |  |  | 37,474 |  |  | — |
Deferral power costs | 9,792 |  |  | 30,219 |  |  | 5,816 |  |  | 24,057 |
Decoupling rebate | 8,853 |  |  | 426 |  |  | — |  |  | 5,816 |
Utility plant retirement costs | — |  |  | 293,965 |  |  | — |  |  | 285,786 |
Interest rate swaps | — |  |  | 36,345 |  |  | — |  |  | 18,638 |
Other regulatory liabilities | 4,041 |  |  | 3,771 |  |  | 4,974 |  |  | 5,250 |
Total regulatory liabilities | $ | 94,978 |  |  | $ | 798,440 |  |  | $ | 48,264 |  |  | $ | 800,089 |

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- Definition Balance Sheet Components [Abstract] + References No definition available. + Details
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- Definition The entire disclosure for supplemental balance sheet disclosures, including descriptions and amounts for assets, liabilities, and equity. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -URI http://asc.fasb.org/topic&trid=2122208 + Details
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