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Balance Sheet Components Balance Sheet Components (Tables) | 9 Months Ended
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Sep. 30, 2018
Balance Sheet Components [Abstract] |
Schedule of Inventory, Current [Table Text Block] | 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 |
Public Utility Property, Plant, and Equipment [Table Text Block] | 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 [Table Text Block] | 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 |
Schedule Of Regulated Asset And Liability [Table Text Block] | 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 Tabular disclosure of the carrying amount as of the balance sheet date of merchandise, goods, commodities, or supplies held for future sale or to be used in manufacturing, servicing or production process. + 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(6)(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: 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(6)(a)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6361739&loc=d3e7789-107766 Reference 4: 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(6)(c)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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- Definition Tabular disclosure of public utility physical assets used in the normal conduct of business and not intended for resale. Includes, but is not limited to, balances by class of assets, deprecation expense and method used, including composite depreciation, and accumulated deprecation. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 20 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=84167750&loc=d3e42232-110370 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.13(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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