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		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Numerator:
Net income attributable to Avista Corp. shareholders	$ 17,453	$ 25,319	$ 65,877	$ 141,113
Denominator:
Weighted-average number of common shares outstanding-basic	67,481	65,894	67,360	65,814
Effect of dilutive securities:
Performance and restricted stock awards	108	69	125	69
Weighted-average number of common shares outstanding-diluted	67,589	65,963	67,485	65,883
Earnings per common share attributable to Avista Corp. shareholders:
Basic	$ 0.26	$ 0.38	$ 0.98	$ 2.14
Diluted	$ 0.26	$ 0.38	$ 0.98	$ 2.14