Fair Value (Reconciliation For Energy Commodity Derivative Assets (Liabilities Measured At Fair Value On A Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2012		Mar. 31, 2011
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Balance as of January 1	$ (12,858)		$ 13,459
Included in regulatory assets/liabilities	(10,351)	[1]	(86)	[1]
Settlements	1,296		956
Ending balance as of March 31	(21,913)		14,329
Natural Gas Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Balance as of January 1	(1,688)
Included in net income
Included in other comprehensive income
Included in regulatory assets/liabilities	290	[1]		[1]
Purchases
Issuances
Settlements	(956)
Transfers to other categories
Ending balance as of March 31	(2,354)
Power Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Balance as of January 1	(9,910)		15,793
Included in regulatory assets/liabilities	(10,914)	[1]	(887)	[1]
Settlements	2,252		956
Ending balance as of March 31	(18,572)		15,862
Power Option Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Balance as of January 1	(1,260)		(2,334)
Included in regulatory assets/liabilities	273	[1]	801	[1]
Ending balance as of March 31	$ (987)		$ (1,533)

[1] The WUTC and the IPUC issued accounting orders authorizing Avista Utilities to offset commodity derivative assets or liabilities with a regulatory asset or liability. This accounting treatment is intended to defer the recognition of mark-to-market gains and losses on energy commodity transactions until the period of settlement. The orders provide for Avista Utilities to not recognize the unrealized gain or loss on utility derivative commodity instruments in the Condensed Consolidated Statements of Income. Realized gains or losses are recognized in the period of settlement, subject to approval for recovery through retail rates. Realized gains and losses, subject to regulatory approval, result in adjustments to retail rates through purchased gas cost adjustments, the ERM in Washington, the PCA mechanism in Idaho, and periodic general rates cases.