Information By Business Segments (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($) Reportable_Segments	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Acquisition costs	$ 965	$ 6,730	$ 2,620	$ 8,004
Number of Reportable Segments | Reportable_Segments			2
Operating revenues	296,013	297,096	$ 1,024,672	1,048,067
Resource costs	101,519	108,568	362,106	376,905
Other operating expenses	86,707	89,255	260,105	255,079
Depreciation and amortization	46,242	43,105	137,006	128,078
Income from operations	36,444	32,899	183,929	208,272
Interest expense	24,605	24,171	75,106	71,771
Income taxes	1,548	5,153	17,467	51,548
Net income from continuing operations attributable to Avista Corp. shareholders	10,119	4,451	90,586	88,338
Payments to Acquire Other Property, Plant, and Equipment	113,340	111,189	297,024	289,072
Total assets	5,557,398		5,557,398		$ 5,514,732
Avista Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	279,549		970,525
Alaska Electric Light & Power [Member]
Segment Reporting Information [Line Items]
Operating revenues	9,570		33,715
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	6,894		20,432
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0	0	0
Resource costs	0	0	0	0
Other operating expenses	0	0	0	0
Depreciation and amortization	0	0	0	0
Income from operations	0	0	0	0
Interest expense	(313)	(71)	(712)	(112)
Income taxes	0	0	0	0
Net income from continuing operations attributable to Avista Corp. shareholders	0	0	0	0
Payments to Acquire Other Property, Plant, and Equipment	0	0	0	0
Total assets	(25,885)		(25,885)		(15,075)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	289,119	291,640	1,004,240	1,030,906
Resource costs	101,519	108,568	362,106	376,905
Other operating expenses	79,360	82,657	239,391	235,216
Depreciation and amortization	46,035	42,968	136,419	127,596
Income from operations	37,104	34,178	184,798	211,456
Interest expense	24,456	24,027	74,639	71,325
Income taxes	2,752	6,016	19,814	53,463
Net income from continuing operations attributable to Avista Corp. shareholders	12,759	5,846	97,605	91,584
Payments to Acquire Other Property, Plant, and Equipment	113,083	110,139	296,215	287,853
Total assets	5,503,857		5,503,857		5,456,566
Operating Segments [Member] | Avista Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	279,549	280,776	970,525	992,904
Resource costs	98,461	104,516	353,148	366,590
Other operating expenses	76,355	79,188	230,342	225,980
Depreciation and amortization	44,569	41,516	132,022	123,249
Income from operations	35,317	32,880	174,310	199,376
Interest expense	23,560	23,132	71,953	68,641
Income taxes	2,564	5,972	17,716	49,881
Net income from continuing operations attributable to Avista Corp. shareholders	11,935	5,419	91,727	85,623
Payments to Acquire Other Property, Plant, and Equipment	108,907	109,066	288,046	283,081
Total assets	5,223,031		5,223,031		5,177,878
Operating Segments [Member] | Alaska Electric Light & Power [Member]
Segment Reporting Information [Line Items]
Operating revenues	9,570	10,864	33,715	38,002
Resource costs	3,058	4,052	8,958	10,315
Other operating expenses	3,005	3,469	9,049	9,236
Depreciation and amortization	1,466	1,452	4,397	4,347
Income from operations	1,787	1,298	10,488	12,080
Interest expense	896	895	2,686	2,684
Income taxes	188	44	2,098	3,582
Net income from continuing operations attributable to Avista Corp. shareholders	824	427	5,878	5,961
Payments to Acquire Other Property, Plant, and Equipment	4,176	1,073	8,169	4,772
Total assets	280,826		280,826		278,688
Operating Segments [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	6,894	5,456	20,432	17,161
Resource costs	0	0	0	0
Other operating expenses	7,347	6,598	20,714	19,863
Depreciation and amortization	207	137	587	482
Income from operations	(660)	(1,279)	(869)	(3,184)
Interest expense	462	215	1,179	558
Income taxes	(1,204)	(863)	(2,347)	(1,915)
Net income from continuing operations attributable to Avista Corp. shareholders	(2,640)	(1,395)	(7,019)	(3,246)
Payments to Acquire Other Property, Plant, and Equipment	257	1,050	809	1,219
Total assets	$ 79,426		$ 79,426		$ 73,241
Restatement Adjustment [Member]
Segment Reporting Information [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		$ 1,900		$ 5,700