Earnings Per Common Share Attributable To Avista Corporation (Computation Of Basic And Diluted Earnings Per Common Share) (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Numerator:
Net income attributable to Avista Corporation	$ 5,786	$ 10,702	$ 62,352	$ 75,621
Noncontrolling earnings adjustment for dilutive securities	(16)	(170)	(25)	(340)
Adjusted net income attributable to Avista Corporation for computation of diluted earnings per common share	$ 5,770	$ 10,532	$ 62,327	$ 75,281
Denominator:
Weighted-average number of common shares outstanding-basic	59,047	58,057	58,778	57,731
Effect of dilutive securities:
Performance and restricted stock awards	71	131	234	149
Stock options	5	44	14	54
Weighted-average number of common shares outstanding-diluted	59,123	58,232	59,026	57,934
Earnings per common share attributable to Avista Corporation:
Basic	$ 0.10	$ 0.18	$ 1.06	$ 1.31
Diluted	$ 0.10	$ 0.18	$ 1.06	$ 1.30