Selected Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Information [Abstract]
Operating revenues from continuing operations	$ 411,846	$ 301,558	$ 312,580	$ 446,578	$ 404,280	$ 289,477	$ 307,488	$ 440,499	$ 1,472,562	$ 1,441,744	$ 1,391,338
Operating expenses	345,093	268,796	249,849	356,236	340,126	259,650	252,518	358,361	1,219,974	1,210,655	1,204,240
Income from continuing operations	66,753	32,762	62,731	90,342	64,154	29,827	54,970	82,138	252,588	231,089	187,098
Income (Loss) from Continuing Operations, Including Portion Attributable to Noncontrolling Interest	30,604	10,526	31,270	47,466	30,392	8,483	24,239	41,219	119,866	104,333	76,803
Income (Loss) from Discontinued Operations, Net of Tax, Including Portion Attributable to Noncontrolling Interest	1,639	(55)	69,312	1,515	1,140	3,448	1,491	1,882	72,411	7,961	1,997
Net income	32,243	10,471	100,582	48,981	31,532	11,931	25,730	43,101	192,277	112,294	78,800
Net loss (income) attributable to noncontrolling interests	(23)	(20)	289	(482)	134	(518)	(73)	(760)
Net income attributable to Avista Corporation shareholders	32,220	10,451	100,871	48,499	31,666	11,413	25,657	42,341	192,041	111,077	78,210
Income (Loss) from Continuing Operations Attributable to Parent	30,581	10,506	31,254	47,476	30,391	8,450	24,212	41,220	119,817	104,273	76,719
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	$ 1,639	$ (55)	$ 69,617	$ 1,023	$ 1,275	$ 2,963	$ 1,445	$ 1,121	$ 72,224	$ 6,804	$ 1,491
Weighted average, basic	62,290	63,934	60,184	60,122	60,037	59,994	59,937	59,866	61,632	59,960	59,028
Weighted average, diluted	62,671	64,244	60,463	60,168	60,087	60,032	59,962	59,898	61,887	59,997	59,201
Income (Loss) from Continuing Operations, Per Diluted Share	$ 0.48	$ 0.16	$ 0.52	$ 0.79	$ 0.51	$ 0.14	$ 0.40	$ 0.69	$ 1.93	$ 1.74	$ 1.30
Income (Loss) from Discontinued Operations and Disposal of Discontinued Operations, Net of Tax, Per Diluted Share	$ 0.03	$ 0.00	$ 1.15	$ 0.02	$ 0.02	$ 0.05	$ 0.03	$ 0.02	$ 1.17	$ 0.11	$ 0.02
Diluted (usd per share)	$ 0.51	$ 0.16	$ 1.67	$ 0.81	$ 0.53	$ 0.19	$ 0.43	$ 0.71	$ 3.10	$ 1.85	$ 1.32