Selected Quarterly Financial Data (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Information [Abstract]
Non-service portion of pension and other postretirement benefit expenses					$ 2,000	$ 1,900	$ 1,800	$ 2,000	$ 5,156	$ 7,670	$ 10,058
Operating revenues	$ 372,221	$ 296,013	$ 319,298	$ 409,361	397,862	297,096	314,501	436,470	1,396,893	1,445,929	1,442,483
Operating expenses	295,037	259,569	266,019	315,155	313,955	264,197	256,555	319,043	1,135,780	1,153,750	1,142,622
Income from operations	77,184	36,444	53,279	94,206	83,907	32,899	57,946	117,427	261,113	292,179	299,861
Net income	45,869	10,129	25,644	54,956	27,615	4,458	21,722	62,137	136,598	115,932	137,316
Net income attributable to noncontrolling interests	(26)	(10)	(67)	(66)	(37)	(7)	49	(21)	(169)	(16)	(88)
Net income attributable to Avista Corp. shareholders	$ 45,843	$ 10,119	$ 25,577	$ 54,890	$ 27,578	$ 4,451	$ 21,771	$ 62,116	$ 136,429	$ 115,916	$ 137,228
Weighted average, basic	65,688	65,688	65,677	65,639	64,809	64,412	64,401	64,362	65,673	64,496	63,508
Weighted average, diluted	65,846	66,026	65,983	65,931	65,308	64,892	64,553	64,469	65,946	64,806	63,920
Earnings Per Share, Diluted	$ 0.70	$ 0.15	$ 0.39	$ 0.83	$ 0.42	$ 0.07	$ 0.34	$ 0.96	$ 2.07	$ 1.79	$ 2.15