Long-Term Debt- Additional Information (Details) - First Mortgage [Member] $ in Millions	Dec. 31, 2020 USD ($)
Avista Utilities [Member]
Debt Instrument [Line Items]
Amount of First Mortgage Bonds that Could be Issued, Percent	66.66%
Amount of First Mortgage Bonds that Could be Issued	$ 1,700.0
Alaska Electric Light & Power [Member]
Debt Instrument [Line Items]
Amount of First Mortgage Bonds that Could be Issued, Percent	66.66%
Amount of First Mortgage Bonds that Could be Issued	$ 36.1