Business Acquisitions Pro Forma Operating Results (Details) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	3 Months Ended		9 Months Ended		15 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014
Business Acquisition [Line Items]
Revenues Excluding Acquired Entity	$ 292,334,000	$ 289,477,000	$ 1,051,492,000	$ 1,037,464,000
Supplemental Pro Forma Revenues	301,558,000	289,477,000	1,060,716,000	1,037,464,000
Business Acquisition, Pro Forma Revenue	301,558,000	298,786,000	1,086,811,000	1,068,707,000
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual	9,224,000	0	9,224,000	0
Income (Loss) from Continuing Operations Attributable to Parent	9,982,000	8,450,000	88,712,000	73,882,000
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	(55,000)	2,963,000	70,585,000	5,529,000
Net income attributable to Avista Corporation shareholders	10,451,000	11,413,000	159,821,000	79,411,000
Business Acquisition, Pro Forma Net Income (Loss)	10,852,000	11,678,000	166,286,000	85,217,000
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual	524,000	0	524,000	0
Weighted Average Basic Shares Outstanding, Pro Forma	63,934	64,494	64,413	64,433
Pro Forma Weighted Average Shares Outstanding, Diluted	64,244	64,532	64,625	64,464
Business Acquisition, Pro Forma Earnings Per Share, Basic	$ 0.17	$ 0.18	$ 2.58	$ 1.32
Business Acquisition, Pro Forma Earnings Per Share, Diluted	$ 0.17	$ 0.18	$ 2.57	$ 1.32
Alaska Energy Resources Company [Member]
Business Acquisition [Line Items]
Gain (Loss) on Disposition of Assets				2,300,000
Supplemental Pro Forma Revenues	9,224,000	9,309,000	35,319,000	31,243,000
Business Combination, Acquisition Related Costs					2,900,000
Net income attributable to Avista Corporation shareholders	524,000	199,000	6,151,000	7,634,000
Acquisition-related Costs [Member] | Alaska Energy Resources Company [Member]
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs	$ 401,000	$ 66,000	$ 838,000	$ (1,828,000)