Business Acquisitions Pro Forma Operating Results (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		27 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015
Business Acquisition [Line Items]
Revenues Excluding Acquired Entity	$ 304,342	$ 292,334	$ 1,065,126	$ 1,051,492
Supplemental Pro Forma Revenues	313,649	301,558	1,097,471	1,060,716
Business Acquisition, Pro Forma Revenue	313,649	301,558	1,097,471	1,086,811
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual	9,307	9,224	32,345	9,224
Income (Loss) from Continuing Operations Attributable to Parent	12,422	9,982	80,562	88,712
Income (Loss) from Discontinued Operations, Net of Tax, Attributable to Parent	289	(55)	485	70,585
Net income attributable to Avista Corporation shareholders	13,011	10,451	84,706	159,821
Business Acquisition, Pro Forma Net Income (Loss)	13,011	10,852	84,727	166,286
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual	300	524	3,659	524
Alaska Energy Resources Company [Member]
Business Acquisition [Line Items]
Supplemental Pro Forma Revenues	9,307	9,224	32,345	35,319
Net income attributable to Avista Corporation shareholders	300	524	3,659	6,151
Avista Utilities [Member]
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs					$ 3,000
Avista Utilities [Member] | Acquisition-related Costs [Member]
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs	$ 0	$ 401	$ 21	$ 838
Acquisition-related Costs [Member] | Avista Utilities [Member]
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs					$ 400