Fair Value (Reconciliation For All Assets And Liabilities Measured At Fair Value On A Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) (Fair Value, Inputs, Level 3 [Member], USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Liabilities
Beginning Balance	$ (12,858)		$ 13,459		$ 49,470
Total gains or losses (realized/unrealized):
Included in net income	0		0		0
Included in other comprehensive income	0		0		0
Included in regulatory assets/liabilities	(15,113)	[1]	(24,876)	[1]	(33,734)	[1]
Purchases	0		0		0
Issuances	0		0		0
Settlements	5,420		2,963		(2,277)
Transfers from other categories	0		(4,404)	[2]	0
Ending Balance	(22,551)		(12,858)		13,459
Natural Gas Exchange Agreements [Member]
Total gains or losses (realized/unrealized):
Included in net income	0		0		0
Included in other comprehensive income	0		0		0
Included in regulatory assets/liabilities	343	[1]	2,621	[1]	0	[1]
Purchases	0		0		0
Issuance	0		0		0
Settlements	(1,034)		95		0
Transfers to/from other categories	0		(4,404)	[2]	0
Liabilities
Beginning Balance	(1,688)		0		0
Total gains or losses (realized/unrealized):
Ending Balance	(2,379)		(1,688)		0
Power Exchange Agreements [Member]
Total gains or losses (realized/unrealized):
Included in net income	0		0		0
Included in other comprehensive income	0		0		0
Included in regulatory assets/liabilities	(15,236)	[1]	(28,571)	[1]	(39,180)	[1]
Purchases	0		0		0
Issuance	0		0		0
Settlements	6,454		2,868		(2,277)
Transfers to/from other categories	0		0	[2]	0
Liabilities
Beginning Balance	(9,910)		15,793		57,250
Total gains or losses (realized/unrealized):
Ending Balance	(18,692)		(9,910)		15,793
Power Option Agreement [Member]
Total gains or losses (realized/unrealized):
Included in net income	0		0		0
Included in other comprehensive income	0		0		0
Included in regulatory assets/liabilities	(220)	[1]	1,074	[1]	5,446	[1]
Purchases	0		0		0
Issuance	0		0		0
Settlements	0		0		0
Transfers to/from other categories	0		0	[2]	0
Liabilities
Beginning Balance	(1,260)		(2,334)		(7,780)
Total gains or losses (realized/unrealized):
Ending Balance	$ (1,480)		$ (1,260)		$ (2,334)

[1] The UTC 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 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.

[2] A derivative contract was reclassified from Level 2 to Level 3 during 2011 due to a particular unobservable input becoming more significant to the fair value measurement. There were not any reclassifications between Level 1 and Level 2. The Company's policy is to reclassify identified items as of the end of the reporting period.