Long-Term Debt (Narrative) (Details) (USD $)	0 Months Ended
	Dec. 31, 1998	Sep. 30, 2013		Dec. 31, 2012
Debt Instrument [Line Items]
Long-term Pollution Control Bond, Noncurrent		$ (83,700,000)	[1],[2]	$ (83,700,000)	[1],[2]
Debt instrument, interest rate, stated percentage	8.45%
Amount borrowed to acquire a long-term fixed rate electric capacity contract	145,000,000
Funds held in trust account which are used to secure long-term debt		1,600,000
2032 [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Pollution Control Bond, Noncurrent		66,700,000	[2]	66,700,000	[2]
2034 [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Long-term Pollution Control Bond, Noncurrent		$ 17,000,000	[1]	$ 17,000,000	[1]
2047 [Member] | First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage		4.23%

[1] In December 2010, $17.0 million of the City of Forsyth, Montana Pollution Control Revenue Refunding Bonds, (Avista Corporation Colstrip Project) due 2034, which had been held by Avista Corp. since 2009, were refunded by a new bond issue (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, the 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 Sheet.

[2] In December 2010, $66.7 million of the City of Forsyth, Montana Pollution Control Revenue Refunding Bonds (Avista Corporation Colstrip Project) due 2032, which had been held by Avista Corp. since 2008, were refunded by a new bond issue (Series 2010A). 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 Sheet.