Selected Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012		Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Selected Quarterly Financial Information [Abstract]
Operating revenues	$ 410,528		$ 340,632		$ 343,585		$ 452,257		$ 438,927		$ 343,710		$ 360,557		$ 476,586		$ 1,547,002		$ 1,619,780		$ 1,558,740
Operating expenses	369,481	[1]	314,023	[1]	297,565	[1]	375,863	[1]	380,603	[1]	310,064	[1]	306,917	[1]	394,192	[1]	1,356,932		1,391,776		1,339,006
Income from operations	41,047	[1]	26,609	[1]	46,020	[1]	76,394	[1]	58,324	[1]	33,646	[1]	53,640	[1]	82,394	[1]	190,070	[2]	228,004	[2]	219,734	[2]
Net income	16,093		5,962		18,532		38,213		25,971		11,637		23,528		42,403		78,800		103,539		94,948
Net loss (income) attributable to noncontrolling interests	(235)		(176)		(354)		175		(1,368)		(935)		(527)		(485)
Net income attributable to Avista Corporation shareholders	$ 15,858		$ 5,786		$ 18,178		$ 38,388		$ 24,603		$ 10,702		$ 23,001		$ 41,918		$ 78,210		$ 100,224		$ 92,425
Weighted average, basic	59,774		59,047		58,702		58,581		58,304		58,057		57,787		57,342		59,028		57,872		55,595
Weighted average, diluted	59,826		59,123		58,924		58,950		58,583		58,232		58,143		57,414		59,201		58,092		55,824
Diluted (usd per share)	$ 0.26		$ 0.10		$ 0.31		$ 0.65		$ 0.42		$ 0.18		$ 0.39		$ 0.73		$ 1.32		$ 1.72		$ 1.65

[1] Includes an immaterial correction of an error related to the reclassification of certain operating expenses from other expense-net to other operating expenses. This correction did not have an impact on net income or earnings per share. See Note 1 for further information regarding this reclassification.

[2] Includes an immaterial correction of an error related to the reclassification of certain operating expenses from other expense-net to utility and non-utility other operating expenses and utility taxes other than income taxes. This correction did not have an impact on net income or earnings per share. See Note 1 for further information regarding this reclassification.