Selected Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Selected Quarterly Financial Data [Abstract]
Operating revenues	$ 438,927	$ 343,710	$ 360,557	$ 476,586	$ 374,420	$ 367,172	$ 360,733	$ 456,415	$ 1,619,780	$ 1,558,740	$ 1,512,565
Operating expenses	378,532	308,832	305,033	391,761	311,549	329,428	298,984	388,591	1,384,158	1,328,552	1,311,907
Income from operations	60,395	34,878	55,524	84,825	62,871	37,744	61,749	67,824	235,622	230,188	200,658
Net income	25,971	11,637	23,528	42,403	26,250	13,334	26,047	29,317	103,539	94,948	88,648
Less: Net income attributable to noncontrolling interests	(1,368)	(935)	(527)	(485)	(521)	(988)	(507)	(507)	(3,315)	(2,523)	(1,577)
Net income attributable to Avista Corporation	$ 24,603	$ 10,702	$ 23,001	$ 41,918	$ 25,729	$ 12,346	$ 25,540	$ 28,810	$ 100,224	$ 92,425	$ 87,071
Weighted average, basic	58,304	58,057	57,787	57,342	56,835	55,616	55,031	54,869	57,872	55,595	54,694
Weighted average, diluted	58,583	58,232	58,143	57,414	57,126	55,801	55,231	55,115	58,092	55,824	54,942
Earnings per common share attributable to Avista Corporation, diluted	$ 0.42	$ 0.18	$ 0.39	$ 0.73	$ 0.45	$ 0.22	$ 0.46	$ 0.52	$ 1.72	$ 1.65	$ 1.58