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Regulatory Matters - Schedule of Regulatory Assets and Liabilities (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019
Regulated Asset Liability [Line Items] |  |
Earning a return, asset | $ 448,712 |
Not earning a return, asset | 219,990 |
Pending regulatory treatment, asset | 95,414 |
Regulatory assets | 13,673 | $ 21,851
Non-current regulatory assets | 750,443 | 670,802
Earning a return, liability | 785,088 |
Not earning a return, liability | 35,218 |
Pending Regulatory Treatment Liability | 10,949 |
Regulatory liabilities | 46,435 | 51,715
Non-current regulatory liabilities | 784,820 | 775,436
Natural Gas Deferrals [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, liability | 874 |
Not earning a return, liability | 0 |
Pending Regulatory Treatment Liability | 0 |
Regulatory liabilities | 874 | 3,189
Non-current regulatory liabilities | 0 | 0
Power Deferrals Regulatory Liability [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, liability | 37,869 |
Not earning a return, liability | 0 |
Pending Regulatory Treatment Liability | 0 |
Regulatory liabilities | 20,299 | 14,155
Non-current regulatory liabilities | 17,570 | 23,544
Removal Costs [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, liability | 325,832 |
Not earning a return, liability | 0 |
Pending Regulatory Treatment Liability | 0 |
Regulatory liabilities | 0 | 0
Non-current regulatory liabilities | 325,832 | 312,403
Deferred Income Tax Charge [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, liability | 390,056 |
Not earning a return, liability | 24,573 |
Pending Regulatory Treatment Liability | 0 |
Regulatory liabilities | 14,952 | 23,803
Non-current regulatory liabilities | 399,677 | 407,549
Interest Rate Swaps [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, liability | 15,046 |
Not earning a return, liability | 0 |
Pending Regulatory Treatment Liability | 0 |
Regulatory liabilities | 0 | 0
Non-current regulatory liabilities | $ 15,046 | 17,088
Decoupling Surcharge [Member] |  |
Regulated Asset Liability [Line Items] |  |
Remaining amortization period, regulatory liability | 2 years |
Earning a return, liability | $ 2,966 |
Not earning a return, liability | 0 |
Pending Regulatory Treatment Liability | 0 |
Regulatory liabilities | 1,447 | 255
Non-current regulatory liabilities | 1,519 | 2,398
Other Regulatory Assets (Liabilities) [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, liability | 12,445 |
Not earning a return, liability | 10,645 |
Pending Regulatory Treatment Liability | 0 |
Regulatory liabilities | 8,863 | 10,313
Non-current regulatory liabilities | 14,227 | 12,454
COVID-19 Deferrals [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, liability | 0 |
Not earning a return, liability | 0 |
Pending Regulatory Treatment Liability | 10,949 |
Regulatory liabilities | 0 | 0
Non-current regulatory liabilities | 10,949 | 0
Deferred Income Tax Charge [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, asset | 108,517 |
Not earning a return, asset | 0 |
Pending regulatory treatment, asset | 0 |
Regulatory assets | 0 | 0
Non-current regulatory assets | 108,517 | 95,752
Pension and Other Postretirement Benefit Plans [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, asset | 0 |
Not earning a return, asset | 198,746 |
Pending regulatory treatment, asset | 0 |
Regulatory assets | 0 | 0
Non-current regulatory assets | 198,746 | 208,754
Energy commodity derivatives |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, asset | 0 |
Not earning a return, asset | 7,795 |
Pending regulatory treatment, asset | 0 |
Regulatory assets | 2,073 | 6,310
Non-current regulatory assets | 5,722 | 264
Unamortized Debt Repurchase Costs [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, asset | 7,512 |
Not earning a return, asset | 0 |
Pending regulatory treatment, asset | 0 |
Regulatory assets | 0 | 0
Non-current regulatory assets | $ 7,512 | 8,884
Settlement with Coeur d'Alene Tribe [Member] |  |
Regulated Asset Liability [Line Items] |  |
Remaining amortization period, regulatory assets | 39 years |
Earning a return, asset | $ 40,043 |
Not earning a return, asset | 0 |
Pending regulatory treatment, asset | 0 |
Regulatory assets | 0 | 0
Non-current regulatory assets | 40,043 | 41,332
Demand Side Management Programs [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, asset | 0 |
Not earning a return, asset | 3,814 |
Pending regulatory treatment, asset | 0 |
Regulatory assets | 0 | 0
Non-current regulatory assets | $ 3,814 | 12,170
Decoupling Surcharge [Member] |  |
Regulated Asset Liability [Line Items] |  |
Remaining amortization period, regulatory assets | 2 years |
Earning a return, asset | $ 24,246 |
Not earning a return, asset | 0 |
Pending regulatory treatment, asset | 0 |
Regulatory assets | 7,123 | 12,098
Non-current regulatory assets | 17,123 | 14,806
Utility Plant to be Abandoned [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, asset | 28,916 |
Not earning a return, asset | 0 |
Pending regulatory treatment, asset | 0 |
Regulatory assets | 0 | 0
Non-current regulatory assets | 28,916 | 31,291
Interest Rate Swaps [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, asset | 130,538 |
Not earning a return, asset | 0 |
Pending regulatory treatment, asset | 84,313 |
Regulatory assets | 0 | 0
Non-current regulatory assets | 214,851 | 168,594
Deferred power costs [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, asset | 3,337 |
Not earning a return, asset | 0 |
Pending regulatory treatment, asset | 0 |
Regulatory assets | 1,775 | 0
Non-current regulatory assets | 1,562 | 0
AFUDC Above FERC Allowed Rate [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, asset | 47,393 |
Not earning a return, asset | 0 |
Pending regulatory treatment, asset | 0 |
Regulatory assets | 0 | 0
Non-current regulatory assets | 47,393 | 40,749
Other Regulatory Assets (Liabilities) [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, asset | 31,831 |
Not earning a return, asset | 9,635 |
Pending regulatory treatment, asset | 2,935 |
Regulatory assets | 2,702 | 3,443
Non-current regulatory assets | 41,699 | 34,811
COVID-19 Deferrals [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, asset | 0 |
Not earning a return, asset | 0 |
Pending regulatory treatment, asset | 8,166 |
Regulatory assets | 0 | 0
Non-current regulatory assets | 8,166 | 0
Advanced Meter Infrastructure [Member] |  |
Regulated Asset Liability [Line Items] |  |
Earning a return, asset | 26,379 |
Not earning a return, asset | 0 |
Pending regulatory treatment, asset | 0 |
Regulatory assets | 0 | 0
Non-current regulatory assets | $ 26,379 | $ 13,395

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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://fasb.org/us-gaap/role/ref/legacyRef -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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