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Balance Sheet Components Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands | Sep. 30, 2018 | Dec. 31, 2017
--- | --- | ---
Regulated Asset Liability [Line Items] |  |
Regulatory assets | $ 21,525 | $ 44,750
Regulatory Assets, Noncurrent | 576,863 | 619,399
Regulatory liabilities | 94,978 | 48,264
Non-current regulatory liabilities | 798,440 | 800,089
Income Tax Related |  |
Regulated Asset Liability [Line Items] |  |
Regulatory liabilities | 36,850 | 0
Non-current regulatory liabilities | 433,714 | 460,542
Natural Gas Deferrals [Member] |  |
Regulated Asset Liability [Line Items] |  |
Regulatory liabilities | 35,442 | 37,474
Non-current regulatory liabilities | 0 | 0
Power Deferrals Regulatory Liability [Member] |  |
Regulated Asset Liability [Line Items] |  |
Regulatory liabilities | 9,792 | 5,816
Non-current regulatory liabilities | 30,219 | 24,057
Decoupling surcharge |  |
Regulated Asset Liability [Line Items] |  |
Regulatory liabilities | 8,853 | 0
Non-current regulatory liabilities | 426 | 5,816
Removal Costs [Member] |  |
Regulated Asset Liability [Line Items] |  |
Regulatory liabilities | 0 | 0
Non-current regulatory liabilities | 293,965 | 285,786
Interest rate swaps |  |
Regulated Asset Liability [Line Items] |  |
Regulatory liabilities | 0 | 0
Non-current regulatory liabilities | 36,345 | 18,638
Other Regulatory Assets (Liabilities) [Member] |  |
Regulated Asset Liability [Line Items] |  |
Regulatory liabilities | 4,041 | 4,974
Non-current regulatory liabilities | 3,771 | 5,250
Energy commodity derivatives |  |
Regulated Asset Liability [Line Items] |  |
Regulatory assets | 19,238 | 24,991
Regulatory Assets, Noncurrent | 9,555 | 18,967
Decoupling surcharge |  |
Regulated Asset Liability [Line Items] |  |
Regulatory assets | 2,287 | 19,759
Regulatory Assets, Noncurrent | 18,099 | 2,600
Pension and other postretirement benefit plans |  |
Regulated Asset Liability [Line Items] |  |
Regulatory assets | 0 | 0
Regulatory Assets, Noncurrent | 201,620 | 209,115
Interest rate swaps |  |
Regulated Asset Liability [Line Items] |  |
Regulatory assets | 0 | 0
Regulatory Assets, Noncurrent | 131,972 | 169,704
Income Tax Related |  |
Regulated Asset Liability [Line Items] |  |
Regulatory assets | 0 | 0
Regulatory Assets, Noncurrent | 90,285 | 90,315
Settlement with Coeur d'Alene Tribe |  |
Regulated Asset Liability [Line Items] |  |
Regulatory assets | 0 | 0
Regulatory Assets, Noncurrent | 42,971 | 43,954
Demand side management programs |  |
Regulated Asset Liability [Line Items] |  |
Regulatory assets | 0 | 0
Regulatory Assets, Noncurrent | 19,774 | 24,620
Utility plant to be abandoned |  |
Regulated Asset Liability [Line Items] |  |
Regulatory assets | 0 | 0
Regulatory Assets, Noncurrent | 24,158 | 24,330
Other Regulatory Assets (Liabilities) [Member] |  |
Regulated Asset Liability [Line Items] |  |
Regulatory assets | 0 | 0
Regulatory Assets, Noncurrent | $ 38,429 | $ 35,794

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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are expected to be recovered through revenue sources within one year or the normal operating cycle, if longer. Such costs are capitalized if they meet both of the following criteria: a. It is probable that future revenue in an amount at least equal to the capitalized cost will result from inclusion of that cost in allowable costs for rate-making purposes. b. Based on available evidence, the future revenue will be provided to permit recovery of the previously incurred cost rather than to provide for expected levels of similar future costs. If the revenue will be provided through an automatic rate-adjustment clause, this criterion requires that the regulator's intent clearly be to permit recovery of the previously incurred cost. + 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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- Definition Carrying amount as of the balance sheet date of capitalized costs of regulated entities that are not expected to be recovered through revenue sources within one year or the normal operating cycle if longer. + 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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- Definition The amount for the individual regulatory current liability as itemized in a table of regulatory current 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 980 -SubTopic 405 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6500807&loc=d3e48068-110394 + Details
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