Business Acquisitions Business Acquisitions Assets Acquired Liabilities Assumed (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended			18 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	Dec. 31, 2014	Jul. 01, 2014
Business Acquisition [Line Items]
Goodwill	$ 57,976				$ 76,257				$ 57,976	$ 76,257	$ 75,959	$ 57,976
Revenues Excluding Acquired Entity									1,450,918	1,441,744
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
Business Acquisition, Pro Forma Revenue									1,497,385	1,483,338
Business Combination, Pro Forma Information, Revenue of Acquiree since Acquisition Date, Actual									21,644	0
Income Loss From Continuing Operations Excluding Acquired Entity									116,665	104,273
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
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
Business Acquisition, Pro Forma Net Income (Loss)									198,565	119,535
Business Combination, Pro Forma Information, Earnings or Loss of Acquiree since Acquisition Date, Actual									3,152	0
Alaska Energy Resources Company [Member]
Business Acquisition [Line Items]
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Cash and Equivalents													19,704
Goodwill													52,730
Operating revenues from continuing operations									46,467	41,594
Business Combination, Acquisition Related Costs												3,000
Net income attributable to Avista Corporation shareholders									8,806	9,328
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Receivables													3,851
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Inventory													2,017
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets, Other													999
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Current Assets													26,571
Business Combination Recognized Identifiable Assets Acquired And Liabilities Assumed Public Utilities Property Plant and Equipment Plant in Service													113,964
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed Capital Lease Assets													71,007
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Public Utilities Property Plant and Equipment, Construction Work in Progress													3,440
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Property, Plant, and Equipment													188,411
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed, Noncurrent Non-Utility Property													6,660
Business Combination, Recognized Identifiable Assets Acquired And Liabilities Assumed Electric Plant Held for Future Use													3,711
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Other Noncurrent Assets													5,368
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Noncurrent Assets													68,469
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets													283,451
Acquisition-related Costs [Member] | Alaska Energy Resources Company [Member]
Business Acquisition [Line Items]
Business Combination, Acquisition Related Costs									$ 870	$ 870