Long-Term Debt (Narrative) (Details) - USD ($) $ in Thousands		3 Months Ended		6 Months Ended
		Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014	Aug. 18, 1998
Debt Instrument [Line Items]
Capital Lease Obligations		$ 73,422		$ 73,422		$ 74,149
Long-term Pollution Control Bond, Noncurrent	[1]	$ (83,700)		$ (83,700)		$ (83,700)
Debt instrument, interest rate, stated percentage		1.16%		1.16%		1.11%
Utilities Operating Expense, Products and Services		$ 141,116	$ 128,922	$ 350,676	$ 349,419
2032 [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Pollution Control Bond, Noncurrent		66,700		66,700
2034 [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Pollution Control Bond, Noncurrent		17,000		17,000
AIDEA [Member]
Debt Instrument [Line Items]
Long Term Revenue Bonds							$ 100,000
Alaska Electric Light & Power [Member]
Debt Instrument [Line Items]
Capital Leased Assets, Gross		71,007		71,007		$ 71,007
Capital Leases Assets Accumulated Depreciation		3,641		3,641		$ 1,821
Alaska Electric Light & Power [Member] | Capital Lease Obligations [Member]
Debt Instrument [Line Items]
Capital Lease Obligations		$ 68,840		68,840
Capital Lease Obligations Annual Minimum Payments of Principal and Interest				5,900
Power purchase agreement [Member] | Alaska Electric Light & Power [Member]
Debt Instrument [Line Items]
Utilities Operating Expense, Products and Services				$ 10,600
Minimum [Member] | AIDEA [Member] | Long-Term Revenue Bonds [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage							4.90%
Maximum [Member] | AIDEA [Member] | Long-Term Revenue Bonds [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage							6.00%

[1] In December 2010, $66.7 million and $17.0 million of the City of Forsyth, Montana Pollution Control Revenue Refunding Bonds (Avista Corporation Colstrip Project) due in 2032 and 2034, respectively, which had been held by Avista Corp. since 2008 and 2009, respectively, were refunded by new bond issues (Series 2010A and Series 2010B). The new bonds were not offered to the public and were purchased by Avista Corp. due to market conditions. The Company expects that at a later date, subject to market conditions, these bonds may be remarketed to unaffiliated investors. So long as Avista Corp. is the holder of these bonds, the bonds will not be reflected as an asset or a liability on Avista Corp.'s Condensed Consolidated Balance Sheets.