Long-Term Debt (Schedule Of Long-Term Debt Instruments) (Details) (USD $) In Thousands, unless otherwise specified							9 Months Ended			9 Months Ended			9 Months Ended		9 Months Ended				9 Months Ended		9 Months Ended			9 Months Ended		9 Months Ended			9 Months Ended			9 Months Ended		9 Months Ended				9 Months Ended					9 Months Ended			9 Months Ended			9 Months Ended
	Oct. 31, 2011	Sep. 30, 2011		Dec. 31, 2010		Sep. 30, 2011 2012 [Member]	Sep. 30, 2011 2012 [Member] Secured Debt [Member]	Dec. 31, 2010 2012 [Member] Secured Debt [Member]	Sep. 30, 2011 2013 [Member]	Sep. 30, 2011 2013 [Member] First Mortgage Bonds [Member]	Dec. 31, 2010 2013 [Member] First Mortgage Bonds [Member]	Sep. 30, 2011 2018, 5.95% [Member]	Sep. 30, 2011 2018, 5.95% [Member] First Mortgage Bonds [Member]	Dec. 31, 2010 2018, 5.95% [Member] First Mortgage Bonds [Member]	Sep. 30, 2011 2018, 7.39% - 7.45% [Member]	Sep. 30, 2011 2018, 7.39% - 7.45% [Member] Secured Debt [Member]	Dec. 31, 2010 2018, 7.39% - 7.45% [Member] Secured Debt [Member]	Sep. 30, 2011 2019 [Member]	Sep. 30, 2011 2019 [Member] First Mortgage Bonds [Member]	Dec. 31, 2010 2019 [Member] First Mortgage Bonds [Member]	Sep. 30, 2011 2020 [Member] First Mortgage Bonds [Member]	Dec. 31, 2010 2020 [Member] First Mortgage Bonds [Member]	Sep. 30, 2011 2022 [Member]	Sep. 30, 2011 2022 [Member] First Mortgage Bonds [Member]	Dec. 31, 2010 2022 [Member] First Mortgage Bonds [Member]	Sep. 30, 2011 2023, 7.18% - 7.54% [Member]	Sep. 30, 2011 2023, 7.18% - 7.54% [Member] Secured Debt [Member]	Dec. 31, 2010 2023, 7.18% - 7.54% [Member] Secured Debt [Member]	Sep. 30, 2011 2023, 6.00% [Member] Unsecured Debt [Member]	Dec. 31, 2010 2023, 6.00% [Member] Unsecured Debt [Member]	Sep. 30, 2011 2028 [Member]	Sep. 30, 2011 2028 [Member] Secured Debt [Member]	Dec. 31, 2010 2028 [Member] Secured Debt [Member]	Sep. 30, 2011 2032 [Member] Secured Debt [Member]		Dec. 31, 2010 2032 [Member] Secured Debt [Member]		Sep. 30, 2011 2034 [Member] Secured Debt [Member]		Dec. 31, 2010 2034 [Member] Secured Debt [Member]		Sep. 30, 2011 2035 [Member]	Sep. 30, 2011 2035 [Member] First Mortgage Bonds [Member]	Dec. 31, 2010 2035 [Member] First Mortgage Bonds [Member]	Sep. 30, 2011 2037 [Member]	Sep. 30, 2011 2037 [Member] First Mortgage Bonds [Member]	Dec. 31, 2010 2037 [Member] First Mortgage Bonds [Member]	Sep. 30, 2011 2040 [Member]	Sep. 30, 2011 2040 [Member] First Mortgage Bonds [Member]	Dec. 31, 2010 2040 [Member] First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Maturity Year							2012			2013			2018			2018			2019		2020			2022			2023		2023			2028		2032	[1]			2034	[2]				2035			2037			2040
Debt instrument, interest rate, stated percentage	4.45%	8.45%				7.37%			1.68%			5.95%						5.45%					5.13%						6.00%		6.37%											6.25%			5.70%			5.55%
Interest Rate, minimum															7.39%						3.89%					7.18%
Medium-Term Notes, Noncurrent							$ 7,000	$ 7,000								$ 22,500	$ 22,500										$ 13,500	$ 13,500				$ 25,000	$ 25,000
Secured long-term debt		1,178,700		1,178,700						50,000	50,000		250,000	250,000					90,000	90,000	52,000	52,000		250,000	250,000																		150,000	150,000		150,000	150,000		35,000	35,000
Other long-term debt and capital leases		5,447		5,500
Settled interest rate swaps		(10,849)		(951)
Unamortized debt discount		(1,667)		(1,792)
Total		1,175,731		1,185,557
Pollution Control Bonds		83,700	[1],[2]	83,700	[1],[2]																								4,100	4,100				66,700	[1]	66,700	[1]	17,000	[2]	17,000	[2]
Current portion of long-term debt		(7,370)		(358)
Total long-term debt		$ 1,084,661		$ 1,101,499

[1] 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.

[2] 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.