Consolidated Statements Of Equity And Redeemable Noncontrolling Interests - USD ($) $ in Thousands	Total	Common Stock [Member]	Accumulated Other Comprehensive Income (Loss) [Member]	Retained Earnings [Member]	Noncontrolling Interests [Member]
Beginning Balance (in shares) at Dec. 31, 2015		62,312,651
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Shares issued		1,869,836
Ending Balance (in shares) at Sep. 30, 2016		64,182,487
Beginning Balance at Dec. 31, 2015		$ 1,004,336	$ (6,650)	$ 530,940
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Equity compensation expense		5,462
Issuance of common stock, net of issuance costs		66,756
Payment of minimum tax withholdings for share-based payment awards		(3,073)
Purchase of subsidiary noncontrolling interests		0			$ 0
Other comprehensive income (loss)	$ (949)		(949)
Net income attributable to Avista Corporation shareholders	97,137			97,137
Cash dividends paid (common stock)				(65,172)
Ending Balance at Sep. 30, 2016	$ 1,628,787	$ 1,073,481	(7,599)	562,905
Beginning Balance Noncontrolling Interest at Dec. 31, 2015					(339)
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Net income (loss) attributable to noncontrolling interests					76
Ending Balance Noncontrolling Interest at Sep. 30, 2016					(263)
Beginning Balance (in shares) at Dec. 31, 2015		62,312,651
Ending Balance (in shares) at Dec. 31, 2016	64,187,934	64,187,934
Beginning Balance at Dec. 31, 2015		$ 1,004,336	(6,650)	530,940
Ending Balance at Dec. 31, 2016	$ 1,648,727	$ 1,075,281	(7,568)	581,014
Beginning Balance Noncontrolling Interest at Dec. 31, 2015					(339)
Ending Balance Noncontrolling Interest at Dec. 31, 2016	251				(251)
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Total equity	1,628,524
Total equity	$ 1,648,476
Shares issued		226,638
Ending Balance (in shares) at Sep. 30, 2017	64,414,572	64,414,572
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Equity compensation expense		$ 5,055
Issuance of common stock, net of issuance costs		1,490
Payment of minimum tax withholdings for share-based payment awards		(3,420)
Purchase of subsidiary noncontrolling interests		(1,191)			891
Other comprehensive income (loss)	$ 548		548
Net income attributable to Avista Corporation shareholders	88,338			88,338
Cash dividends paid (common stock)				(69,220)
Ending Balance at Sep. 30, 2017	1,670,327	$ 1,077,215	$ (7,020)	$ 600,132
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Net income (loss) attributable to noncontrolling interests					(21)
Ending Balance Noncontrolling Interest at Sep. 30, 2017	(619)				$ 619
Common Stock, Number of Shares, Par Value and Other Disclosures [Abstract]
Total equity	$ 1,670,946