Information By Business Segments (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) Reportable_Segments	Mar. 31, 2017 USD ($)	Dec. 31, 2017 USD ($)
Segment Reporting Information [Line Items]
Acquisition costs	$ 672	$ 0
Number of Reportable Segments | Reportable_Segments	2
Operating revenues	$ 409,361	436,470
Resource costs	154,618	165,586
Other operating expenses	84,794	78,622
Depreciation and amortization	44,914	42,173
Income from operations	94,206	117,427
Interest expense	25,029	23,730
Income taxes	10,710	33,344
Net income from continuing operations attributable to Avista Corp. shareholders	54,890	62,116
Payments to Acquire Other Property, Plant, and Equipment	82,031	86,798
Total assets	5,485,183		$ 5,514,732
Avista Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	388,754
Alaska Electric Light & Power [Member]
Segment Reporting Information [Line Items]
Operating revenues	13,663
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	6,944
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Operating revenues	0	0
Resource costs	0	0
Other operating expenses	0	0
Depreciation and amortization	0	0
Income from operations	0	0
Interest expense	(165)	(14)
Income taxes	0	0
Net income from continuing operations attributable to Avista Corp. shareholders	0	0
Payments to Acquire Other Property, Plant, and Equipment	0	0
Total assets	(22,951)		(15,075)
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	402,417	430,537
Resource costs	154,618	165,586
Other operating expenses	77,970	72,443
Depreciation and amortization	44,733	41,985
Income from operations	94,267	117,861
Interest expense	24,859	23,577
Income taxes	11,881	33,480
Net income from continuing operations attributable to Avista Corp. shareholders	59,312	62,292
Payments to Acquire Other Property, Plant, and Equipment	81,817	86,763
Total assets	5,432,390		5,456,566
Operating Segments [Member] | Avista Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	388,754	415,381
Resource costs	151,665	162,613
Other operating expenses	75,139	69,671
Depreciation and amortization	43,267	40,538
Income from operations	88,145	110,676
Interest expense	23,965	22,683
Income taxes	10,417	31,017
Net income from continuing operations attributable to Avista Corp. shareholders	55,540	58,439
Payments to Acquire Other Property, Plant, and Equipment	81,176	85,403
Total assets	5,149,906		5,177,878
Operating Segments [Member] | Alaska Electric Light & Power [Member]
Segment Reporting Information [Line Items]
Operating revenues	13,663	15,156
Resource costs	2,953	2,973
Other operating expenses	2,831	2,772
Depreciation and amortization	1,466	1,447
Income from operations	6,122	7,185
Interest expense	894	894
Income taxes	1,464	2,463
Net income from continuing operations attributable to Avista Corp. shareholders	3,772	3,853
Payments to Acquire Other Property, Plant, and Equipment	641	1,360
Total assets	282,484		278,688
Operating Segments [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	6,944	5,933
Resource costs	0	0
Other operating expenses	6,824	6,179
Depreciation and amortization	181	188
Income from operations	(61)	(434)
Interest expense	335	167
Income taxes	(1,171)	(136)
Net income from continuing operations attributable to Avista Corp. shareholders	(4,422)	(176)
Payments to Acquire Other Property, Plant, and Equipment	214	35
Total assets	$ 75,744		$ 73,241
Restatement Adjustment [Member]
Segment Reporting Information [Line Items]
Defined Benefit Plan, Net Periodic Benefit Cost (Credit)		$ 2,000