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Long- Term Debt to Affiliated Trust - Narrative (Details) - USD ($) $ in Millions | 9 Months Ended | 12 Months Ended
--- | --- | ---
Sep. 30, 2021 | Dec. 31, 2000
Debt Instrument [Line Items] |  |
Payments for repurchase of trust preferred securities |  | $ 10.0
1997 Floating Rate Junior Subordinated Deferrable Interest Debentures, Series B [Member] | Avista Capital II [Member] |  |
Debt Instrument [Line Items] |  |
Junior subordinated debenture issuance date | 1997 |
Principal amount | $ 51.5 |
Avista Corp [Member] |  |
Debt Instrument [Line Items] |  |
Noncontrolling interest, ownership percentage by parent | 100.00% |
Trust Preferred Securities Subject to Mandatory Redemption [Member] | 1997 Floating Rate Junior Subordinated Deferrable Interest Debentures, Series B [Member] | Avista Capital II [Member] |  |
Debt Instrument [Line Items] |  |
Principal amount | $ 50.0 |
Debt instrument, description of variable rate basis | LIBOR |
Debt instrument, basis spread on variable rate | 0.875% |
Common Trust Securities [Member] | 1997 Floating Rate Junior Subordinated Deferrable Interest Debentures, Series B [Member] | Avista Capital II [Member] |  |
Debt Instrument [Line Items] |  |
Principal amount | $ 1.5 |

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- Definition The cash outflow to reacquire preferred stocks issued by a business trust or other special purpose entity, mainly established by a bank holding entity, to third party investors. The trust's assets are deeply subordinated debentures of the bank holding entity. Most trust preferred securities are subject to a mandatory redemption upon the repayment of the debentures. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 15 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=123570139&loc=d3e3291-108585 + Details
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