Long-Term Debt and Capital Leases Long-Term Debt and Capital Leases (Details) $ in Thousands	9 Months Ended
Sep. 30, 2018 USD ($) Caontracts	Dec. 31, 2017 USD ($)
Debt Instrument [Line Items]
Long-term Pollution Control Bond, Noncurrent	$ (83,700)	$ (83,700)
First Mortgage Bonds	1,889,200	1,786,700
Secured and Unsecured Debt	1,904,200	1,801,700
Capital Lease Obligations	57,844	62,148
Unamortized Discount	(905)	(626)
Unamortized Long-Term Debt Issuance Costs	(13,866)	(10,285)
Long-Term Debt, Before Current Portion And Bonds Held by Company	1,947,273	1,852,937
Current portion of long-term debt	(2,629)	(277,438)
Long-term debt and capital leases	1,860,944	1,491,799
Payments for (proceeds from) settlement of derivative instruments	(26,600)
Avista Utilities [Member]
Debt Instrument [Line Items]
First Mortgage Bonds	$ 1,814,200	1,711,700
Avista Utilities [Member] \| Secured Debt [Member] \| 2018 7.39% To 7.45%
Debt Instrument [Line Items]
Maturity Year	2018
Secured Medium-term Notes	$ 0	22,500
Avista Utilities [Member] \| Secured Debt [Member] \| 2018 7.39% To 7.45% \| Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.39%
Avista Utilities [Member] \| Secured Debt [Member] \| 2018 7.39% To 7.45% \| Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.45%
Avista Utilities [Member] \| Secured Debt [Member] \| 2023 7.18% To 7.54%
Debt Instrument [Line Items]
Maturity Year	2023
Secured Medium-term Notes	$ 13,500	13,500
Avista Utilities [Member] \| Secured Debt [Member] \| 2023 7.18% To 7.54% \| Minimum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.18%
Avista Utilities [Member] \| Secured Debt [Member] \| 2023 7.18% To 7.54% \| Maximum [Member]
Debt Instrument [Line Items]
Debt Instrument, Interest Rate, Stated Percentage	7.54%
Avista Utilities [Member] \| Secured Debt [Member] \| 2028
Debt Instrument [Line Items]
Maturity Year	2028
Secured Medium-term Notes	$ 25,000	25,000
Debt Instrument, Interest Rate, Stated Percentage	6.37%
Avista Utilities [Member] \| Secured Debt [Member] \| 2032
Debt Instrument [Line Items]
Maturity Year	2032
Long-term Pollution Control Bond, Noncurrent	$ 66,700	66,700
Avista Utilities [Member] \| Secured Debt [Member] \| 2034
Debt Instrument [Line Items]
Maturity Year	2034
Long-term Pollution Control Bond, Noncurrent	$ 17,000	17,000
Avista Utilities [Member] \| First Mortgage [Member] \| 2018 5.95%
Debt Instrument [Line Items]
Maturity Year	2018
Debt Instrument, Interest Rate, Stated Percentage	5.95%
First Mortgage Bonds	$ 0	250,000
Avista Utilities [Member] \| First Mortgage [Member] \| 2019
Debt Instrument [Line Items]
Maturity Year	2019
Debt Instrument, Interest Rate, Stated Percentage	5.45%
First Mortgage Bonds	$ 90,000	90,000
Avista Utilities [Member] \| First Mortgage [Member] \| 2020
Debt Instrument [Line Items]
Maturity Year	2020
Debt Instrument, Interest Rate, Stated Percentage	3.89%
First Mortgage Bonds	$ 52,000	52,000
Avista Utilities [Member] \| First Mortgage [Member] \| 2022
Debt Instrument [Line Items]
Maturity Year	2022
Debt Instrument, Interest Rate, Stated Percentage	5.13%
First Mortgage Bonds	$ 250,000	250,000
Avista Utilities [Member] \| First Mortgage [Member] \| 2035
Debt Instrument [Line Items]
Maturity Year	2035
Debt Instrument, Interest Rate, Stated Percentage	6.25%
First Mortgage Bonds	$ 150,000	150,000
Avista Utilities [Member] \| First Mortgage [Member] \| 2037
Debt Instrument [Line Items]
Maturity Year	2037
Debt Instrument, Interest Rate, Stated Percentage	5.70%
First Mortgage Bonds	$ 150,000	150,000
Avista Utilities [Member] \| First Mortgage [Member] \| 2040
Debt Instrument [Line Items]
Maturity Year	2040
Debt Instrument, Interest Rate, Stated Percentage	5.55%
First Mortgage Bonds	$ 35,000	35,000
Avista Utilities [Member] \| First Mortgage [Member] \| 2041
Debt Instrument [Line Items]
Maturity Year	2041
Debt Instrument, Interest Rate, Stated Percentage	4.45%
First Mortgage Bonds	$ 85,000	85,000
Avista Utilities [Member] \| First Mortgage [Member] \| 2044
Debt Instrument [Line Items]
Maturity Year	2044
Debt Instrument, Interest Rate, Stated Percentage	4.11%
First Mortgage Bonds	$ 60,000	60,000
Avista Utilities [Member] \| First Mortgage [Member] \| 2045
Debt Instrument [Line Items]
Maturity Year	2045
Debt Instrument, Interest Rate, Stated Percentage	4.37%
First Mortgage Bonds	$ 100,000	100,000
Avista Utilities [Member] \| First Mortgage [Member] \| 2047
Debt Instrument [Line Items]
Maturity Year	2047
Debt Instrument, Interest Rate, Stated Percentage	4.23%
First Mortgage Bonds	$ 80,000	80,000
Avista Utilities [Member] \| First Mortgage [Member] \| 2047 3.91%
Debt Instrument [Line Items]
Maturity Year	2047
Debt Instrument, Interest Rate, Stated Percentage	3.91%
First Mortgage Bonds	$ 90,000	90,000
Avista Utilities [Member] \| First Mortgage [Member] \| 2048 4.35%
Debt Instrument [Line Items]
Maturity Year	2048
Debt Instrument, Interest Rate, Stated Percentage	4.35%
First Mortgage Bonds	$ 375,000	0
Avista Utilities [Member] \| First Mortgage [Member] \| 2051
Debt Instrument [Line Items]
Maturity Year	2051
Debt Instrument, Interest Rate, Stated Percentage	3.54%
First Mortgage Bonds	$ 175,000	175,000
Alaska Electric Light & Power [Member] \| First Mortgage [Member] \| 2044
Debt Instrument [Line Items]
Maturity Year	2044
Debt Instrument, Interest Rate, Stated Percentage	4.54%
First Mortgage Bonds	$ 75,000	75,000
Interest Rate Swap [Member] \| 2018
Debt Instrument [Line Items]
Number of interest rate swaps settled \| Caontracts	14
Settled derivative notional amount	$ 275,000
Alaska Energy Resources Company [Member] \| Unsecured Debt [Member] \| 2019
Debt Instrument [Line Items]
Maturity Year	2019
Debt Instrument, Interest Rate, Stated Percentage	3.85%
Unsecured Term Loan	$ 15,000	$ 15,000

Long-Term Debt, Before Current Portion And Bonds Held by Company

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Secured and Unsecured Debt

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Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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Description of the maturity date of the debt instrument including whether the debt matures serially and, if so, a brief description of the serial maturities.

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Noncurrent portion of a debt security issued by a state, certain agencies or authorities, a local government, or development corporation to finance the construction of air- or water-pollution control facilities or sewage or solid waste disposal facilities pursuant to federal law. The bonds are backed by the credit of the beneficiary of the financing rather than the credit of the issuer.

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Carrying value at the balance sheet date of the noncurrent portion of a financial instrument used to receive debt funding on a regular basis with original maturities ranging from 5-10 years.

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

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Including the current and noncurrent portions, carrying value as of the balance sheet date of uncollateralized debt obligations (with maturities initially due after one year or beyond the operating cycle if longer).

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