Long-Term Debt (Schedule Of Long-Term Debt Instruments) (Details) (USD $)	12 Months Ended
	Dec. 31, 2011		Dec. 31, 2010		Dec. 31, 2009	Dec. 31, 1998
Debt Instrument [Line Items]
Debt instrument, interest rate, stated percentage	1.40%		1.17%		1.22%	8.45%
Interest Rate, minimum	1.13%		1.13%		1.22%
Interest Rate, maximum	1.40%		1.41%		3.06%
Secured long-term debt	$ 1,263,700,000		$ 1,178,700,000
Other long-term debt and capital leases	5,455,000		5,500,000
Settled interest rate swaps	(10,629,000)		(951,000)
Unamortized debt discount	(1,626,000)		(1,792,000)
Total	1,261,000,000	[1],[2]	1,185,557,000	[1],[2]
Pollution Control Bonds	83,700,000	[1],[2]	83,700,000	[1],[2]
Current portion of long-term debt	(7,474,000)		(358,000)
Total long-term debt	1,169,826,000		1,101,499,000
2012 [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Maturity Year	2012
Debt instrument, interest rate, stated percentage	7.37%
Medium-Term Notes, Noncurrent	7,000,000		7,000,000
2013 [Member] | First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Maturity Year	2013
Debt instrument, interest rate, stated percentage	1.68%
Secured long-term debt	50,000,000		50,000,000
2018, 5.95% [Member] | First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Maturity Year	2018
Debt instrument, interest rate, stated percentage	5.95%
Secured long-term debt	250,000,000		250,000,000
2018, 7.39% - 7.45% [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Maturity Year	2018
Interest Rate, minimum	7.39%
Interest Rate, maximum	7.45%
Medium-Term Notes, Noncurrent	22,500,000		22,500,000
2019 [Member] | First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Maturity Year	2019
Debt instrument, interest rate, stated percentage	5.45%
Secured long-term debt	90,000,000		90,000,000
2020 [Member] | First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Maturity Year	2020
Debt instrument, interest rate, stated percentage	3.89%
Secured long-term debt	52,000,000		52,000,000
2022 [Member] | First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Maturity Year	2022
Debt instrument, interest rate, stated percentage	5.13%
Secured long-term debt	250,000,000		250,000,000
2023, 7.18% - 7.54% [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Maturity Year	2023
Interest Rate, minimum	7.18%
Interest Rate, maximum	7.54%
Medium-Term Notes, Noncurrent	13,500,000		13,500,000
2023, 6.00% [Member] | Unsecured Debt [Member]
Debt Instrument [Line Items]
Maturity Year	2023
Debt instrument, interest rate, stated percentage	6.00%
Pollution Control Bonds	4,100,000		4,100,000
2028 [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Maturity Year	2028
Debt instrument, interest rate, stated percentage	6.37%
Medium-Term Notes, Noncurrent	25,000,000		25,000,000
2032 [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Maturity Year	2032	[2]
Pollution Control Bonds	66,700,000	[2]	66,700,000	[2]
2034 [Member] | Secured Debt [Member]
Debt Instrument [Line Items]
Maturity Year	2034	[1]
Pollution Control Bonds	17,000,000	[1]	17,000,000	[1]
2035 [Member] | First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Maturity Year	2035
Debt instrument, interest rate, stated percentage	6.25%
Secured long-term debt	150,000,000		150,000,000
2037 [Member] | First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Maturity Year	2037
Debt instrument, interest rate, stated percentage	5.70%
Secured long-term debt	150,000,000		150,000,000
2040 [Member] | First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Maturity Year	2040
Debt instrument, interest rate, stated percentage	5.55%
Secured long-term debt	35,000,000		35,000,000
2041 [Member] | First Mortgage Bonds [Member]
Debt Instrument [Line Items]
Maturity Year	2041	[3]
Debt instrument, interest rate, stated percentage	4.45%	[3]
Secured long-term debt	85,000,000	[3]		[3]
Debt instrument, face amount	$ 85,000,000

[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 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 Consolidated Balance Sheet.

[3] In December 2011, the Company issued $85.0 million of 4.45 percent First Mortgage Bonds due in 2041.