Earnings Per Common Share Attributable to Avista Corp. Shareholders - Schedule of Computation of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Numerator:
Net income attributable to Avista Corp. shareholders	$ 4,876	$ 5,090	$ 70,753	$ 146,203
Denominator:
Weighted-average number of common shares outstanding-basic	68,194	66,265	67,638	65,964
Effect of dilutive securities:
Performance and restricted stock awards	143	86	131	86
Weighted-average number of common shares outstanding-diluted	68,337	66,351	67,769	66,050
Earnings per common share attributable to Avista Corp. shareholders:
Basic	$ 0.07	$ 0.08	$ 1.05	$ 2.22
Diluted	$ 0.07	$ 0.08	$ 1.04	$ 2.21