Earnings Per Common Share Attributable to Avista Corporation Shareholders - Schedule of Computation of Basic and Diluted Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net income attributable to Avista Corp. shareholders	$ 129,488	$ 196,979	$ 136,429
Denominator:
Weighted-average number of common shares outstanding-basic	67,962	66,205	65,673
Effect of dilutive securities:
Performance and restricted stock awards	140	124	273
Weighted-average number of common shares outstanding-diluted	68,102	66,329	65,946
Earnings per common share attributable to Avista Corp. shareholders:
Basic	$ 1.91	$ 2.98	$ 2.08
Diluted	$ 1.90	$ 2.97	$ 2.07