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Regulatory Matters - Additional Information (Details) - USD ($) | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019
Regulated Asset Liability [Line Items] |  |
Threshold limit of deadband and sharing bands for future surcharge or rebate to customer | $ 4,000,000.0 |
Company share of benefit (expense) under washington energy recovery mechanism | 6,200,000 | $ 4,400,000
Threshold to return washington energy recovery mechanism dollars to customers | $ 30,000,000 |
Proposed cumulative rebate refund period | 2 years |
Deferred natural gas costs, asset | $ 1,400,000 |
Deferred natural gas costs, liability |  | 3,200,000
WASHINGTON |  |
Regulated Asset Liability [Line Items] |  |
Decoupling maximum rate increase request | 3.00% |
IDAHO | Power Deferrals Regulatory Asset [Member] |  |
Regulated Asset Liability [Line Items] |  |
Regulatory assets | $ 2,500,000 | 300,000
Power Deferrals Regulatory Liability [Member] | WASHINGTON |  |
Regulated Asset Liability [Line Items] |  |
Regulatory liabilities | 37,900,000 | 37,000,000.0
Revenue Subject to Refund [Member] | WASHINGTON |  |
Regulated Asset Liability [Line Items] |  |
Regulatory liabilities | 0 | 0
Revenue Subject to Refund [Member] | IDAHO |  |
Regulated Asset Liability [Line Items] |  |
Regulatory liabilities | 686,000 | 686,000
Revenue Subject to Refund [Member] | OREGON |  |
Regulated Asset Liability [Line Items] |  |
Regulatory liabilities | $ 0 | $ 0

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- Definition Carrying amount as of the balance sheet date of capitalized gas costs of a regulated entity that are expected to be recoverable through rate adjustments within one year or the normal operating cycle, if longer. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 980 -SubTopic 340 -Section 25 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=121557228&loc=d3e43603-110378 + Details
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