Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected Quarterly Financial Information [Abstract]
Operating revenues	$ 364,559	$ 283,770	$ 300,812	$ 396,481	$ 372,221	$ 296,013	$ 319,298	$ 409,361	$ 1,345,622	$ 1,396,893	$ 1,445,929
Operating expenses	291,252	253,527	261,044	329,410	295,037	259,569	266,019	315,155	1,135,233	1,135,780	1,153,750
Income from operations	73,307	30,243	39,768	67,071	77,184	36,444	53,279	94,206	210,389	261,113	292,179
Net income	50,776	5,090	25,016	115,881	45,869	10,129	25,644	54,956	196,763	136,598	115,932
Net loss (income) attributable to noncontrolling interests	0	0	303	(87)	(26)	(10)	(67)	(66)	216	(169)	(16)
Net income attributable to Avista Corp. shareholders	$ 50,776	$ 5,090	$ 25,319	$ 115,794	$ 45,843	$ 10,119	$ 25,577	$ 54,890	$ 196,979	$ 136,429	$ 115,916
Weighted average, basic	66,929	66,265	65,894	65,733	65,688	65,688	65,677	65,639	66,205	65,673	64,496
Weighted average, diluted	67,059	66,351	65,963	65,941	65,846	66,026	65,983	65,931	66,329	65,946	64,806
Earnings Per Share, Diluted	$ 0.76	$ 0.08	$ 0.38	$ 1.76	$ 0.70	$ 0.15	$ 0.39	$ 0.83	$ 2.97	$ 2.07	$ 1.79