Avista (AVA) 10-Q 1 Nov 17 2017 Q3 Quarterly report Financial data

Document And Entity Information

Document And Entity Information - shares	9 Months Ended
	Sep. 30, 2017	Oct. 30, 2017
Entity Information [Line Items]
Document Type	10-Q
Amendment Flag	false
Document Period End Date	Sep. 30, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	Q3
Trading Symbol	ava
Entity Registrant Name	AVISTA CORP
Entity Central Index Key	104,918
Current Fiscal Year End Date	--12-31
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		64,415,157

Consolidated Statements Of Inco

Consolidated Statements Of Income - USD ($) shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating Revenues:
Utility revenues	$ 291,640	$ 296,989	$ 1,030,906	$ 1,022,670
Other non-utility revenues	5,456	6,360	17,161	17,690
Total operating revenues	297,096	303,349	1,048,067	1,040,360
Utility operating expenses:
Resource costs	108,568	118,737	376,905	390,271
Other operating expenses	77,784	75,160	232,959	229,605
Acquisition costs	6,730	0	8,004	0
Depreciation and amortization	42,968	40,240	127,596	119,110
Taxes other than income taxes	23,269	22,669	79,733	74,669
Other non-utility operating expenses:
Other operating expenses	6,598	6,756	19,863	18,862
Depreciation and amortization	137	193	482	573
Total operating expenses	266,054	263,755	845,542	833,090
Income from operations	31,042	39,594	202,525	207,270
Interest expense	23,955	21,632	71,170	64,223
Interest expense to affiliated trusts	216	164	601	456
Capitalized interest	(899)	(507)	(2,513)	(2,258)
Other income-net	(1,841)	(1,562)	(6,598)	(7,025)
Income before income taxes	9,611	19,867	139,865	151,874
Income tax expense	5,153	7,606	51,548	54,661
Net income	4,458	12,261	88,317	97,213
Net income attributable to noncontrolling interests	(7)	(27)	21	(76)
Net income attributable to Avista Corporation shareholders	$ 4,451	$ 12,234	$ 88,338	$ 97,137
Weighted-average common shares outstanding (thousands), basic	64,412	63,857	64,392	63,282
Weighted-average common shares outstanding (thousands), diluted	64,892	64,325	64,638	63,687
Earnings per common share attributable to Avista Corporation shareholders:
Basic	$ 0.07	$ 0.19	$ 1.37	$ 1.53
Diluted	0.07	0.19	1.37	1.53
Common Stock, Dividends, Per Share, Cash Paid	$ 0.3575	$ 0.3425	$ 1.0725	$ 1.0275

Consolidated Statements Of Comp

Consolidated Statements Of Comprehensive Income - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	$ 4,458	$ 12,261	$ 88,317	$ 97,213
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, after Tax	182	140	548	(949)
Total other comprehensive loss	182	140	548	(949)
Comprehensive Income (Loss), Net of Tax, Including Portion Attributable to Noncontrolling Interest	4,640	12,401	88,865	96,264
Net income attributable to noncontrolling interests	(7)	(27)	21	(76)
Comprehensive Income (Loss), Net of Tax, Attributable to Parent	$ 4,633	$ 12,374	$ 88,886	$ 96,188

Consolidated Statements Of Com4

Consolidated Statements Of Comprehensive Income (Parenthetical) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Other Comprehensive (Income) Loss, Defined Benefit Plan, after Reclassification Adjustment, Tax	$ 98	$ 75	$ 295	$ (512)

Consolidated Balance Sheets

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current Assets:
Cash and cash equivalents	$ 14,716	$ 8,507
Accounts and notes receivable-less allowances of $4,838 and $5,026, respectively	125,595	180,265
Regulatory asset for energy commodity derivatives	22,005	11,365
Materials and supplies, fuel stock and stored natural gas	65,961	53,314
Income taxes receivable	42,111	48,265
Other current assets	46,246	49,625
Total current assets	316,634	351,341
Net Utility Property:
Utility plant in service	5,686,633	5,506,499
Construction work in progress	194,679	150,474
Total	5,881,312	5,656,973
Less: Accumulated depreciation and amortization	1,578,148	1,509,473
Total net utility property	4,303,164	4,147,500
Other Non-current Assets:
Investment in affiliated trusts	11,547	11,547
Goodwill	57,672	57,672
Other property and investments-net	80,022	72,224
Total other non-current assets	149,241	141,443
Deferred Charges:
Regulatory assets for deferred income tax	123,449	109,853
Regulatory assets for pensions and other postretirement benefits	230,988	240,114
Other regulatory assets	129,112	135,751
Regulatory asset for interest rate swaps	170,079	161,508
Non-current regulatory asset for utility derivatives	16,371	16,919
Other deferred charges	13,194	5,326
Total deferred charges	683,193	669,471
Total assets	5,452,232	5,309,755
Current Liabilities:
Accounts payable	72,777	115,545
Current portion of long-term debt	277,626	3,287
Short-term borrowings	106,298	120,000
Utility energy commodity derivative liabilities	11,054	7,035
Accrued interest	29,077	15,869
Accrued taxes other than income taxes	34,520	33,374
Deferred natural gas costs	34,399	30,820
Current portion of pensions and other postretirement benefits	11,544	10,994
Current interest rate swap derivative liabilities	34,520	6,025
Other current liabilities	61,436	64,579
Total current liabilities	673,251	407,528
Long-term debt and capital leases	1,491,789	1,678,717
Long-term debt to affiliated trusts	51,547	51,547
Regulatory liability for utility plant retirement costs	284,263	273,983
Pensions and other postretirement benefits	216,464	226,552
Deferred income taxes	903,943	840,928
Non-current interest rate swap derivative liabilities	1,330	28,705
Other non-current liabilities and deferred credits	158,699	153,319
Total liabilities	3,781,286	3,661,279
Commitments and Contingencies (See Notes to Consolidated Financial Statements)
Avista Corporation Stockholders’ Equity:
Common stock, no par value; 200,000,000 shares authorized; 64,414,572 and 64,187,934 shares issued and outstanding as of September 30, 2017 and December 31, 2016, respectively	1,077,215	1,075,281
Accumulated other comprehensive loss	(7,020)	(7,568)
Retained earnings	600,132	581,014
Total Avista Corporation stockholders’ equity	1,670,327	1,648,727
Noncontrolling Interests	619	(251)
Total equity	1,670,946	1,648,476
Total liabilities and equity	$ 5,452,232	$ 5,309,755

Consolidated Balance Sheets (Pa

Consolidated Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Statement of Financial Position [Abstract]
Accounts and notes receivable, allowances	$ 4,838	$ 5,026
Common stock, par value	$ 0	$ 0
Common stock, shares authorized	200,000,000	200,000,000
Common stock, shares outstanding	64,414,572	64,187,934

Consolidated Statements Of Cash

Consolidated Statements Of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016
Operating Activities:
Net income	$ 88,317	$ 97,213
Non-cash items included in net income:
Depreciation and amortization	130,803	122,414
Provision for deferred income taxes	58,242	87,246
Power and natural gas cost amortizations (deferrals), net	8,416	11,422
Amortization of debt expense	2,440	2,595
Amortization of investment in exchange power	1,838	1,838
Stock-based compensation expense	5,809	6,261
Equity-related AFUDC	(5,012)	(6,306)
Pension and other postretirement benefit expense	27,816	29,076
Amortization of Spokane Energy contract	0	10,904
Other regulatory assets and liabilities and deferred debits and credits	(12,683)	(20,215)
Change in decoupling regulatory deferral	20,193	(24,693)
Other	(190)	5,052
Contributions to defined benefit pension plan	(22,000)	(12,000)
Cash paid for settlement of interest rate swap agreements	(11,302)	(53,966)
Cash received for settlement of interest rate swap agreements	2,479	0
Changes in certain current assets and liabilities:
Accounts and notes receivable	52,534	53,726
Materials and supplies, fuel stock and natural gas stored	(12,653)	(3,932)
Collateral posted for derivative instruments	(1,896)	(19,754)
Income taxes receivable	(4,254)	(25,222)
Other current assets	(16)	(8,486)
Accounts payable	(29,992)	(17,206)
Other current liabilities	8,624	18,151
Net cash provided by operating activities	307,513	254,118
Investing Activities:
Utility property capital expenditures (excluding equity-related AFUDC)	(287,853)	(288,072)
Issuance of notes receivable at subsidiaries	(2,800)	(9,718)
Equity and property investments made by subsidiaries	(10,899)	(8,741)
Distributions received from investments	1,915	0
Other	(2,714)	(8,422)
Net cash used in investing activities	(302,351)	(314,953)
Financing Activities:
Net increase in short-term borrowings	75,000	82,000
Proceeds from issuance of long-term debt	0	70,000
Maturity of long-term debt and capital leases	(2,465)	(92,375)
Issuance of common stock	1,490	66,756
Cash dividends paid	(69,220)	(65,172)
Other	(3,758)	(3,774)
Net cash provided by financing activities	1,047	57,435
Net increase (decrease) in cash and cash equivalents	6,209	(3,400)
Cash and cash equivalents at beginning of period	8,507	10,484
Cash and cash equivalents at end of period	$ 14,716	$ 7,084

Consolidated Statements Of Equi

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

Summary Of Significant Accounti

Summary Of Significant Accounting Policies	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Summary Of Significant Accounting Policies	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Nature of Business Avista Corp. is primarily an electric and natural gas utility with certain other business ventures. Avista Utilities is an operating division of Avista Corp., comprising the regulated utility operations in the Pacific Northwest. Avista Utilities provides electric distribution and transmission, and natural gas distribution services in parts of eastern Washington and northern Idaho. Avista Utilities also provides natural gas distribution service in parts of northeastern and southwestern Oregon. Avista Utilities has electric generating facilities in Washington, Idaho, Oregon and Montana. Avista Utilities also supplies electricity to a small number of customers in Montana, most of whom are employees who operate Avista Utilities' Noxon Rapids generating facility. Alaska Energy and Resources Company (AERC) is a wholly-owned subsidiary of Avista Corp. The primary subsidiary of AERC is Alaska Electric Light and Power Company (AEL&P), which comprises Avista Corp.'s regulated utility operations in Alaska. Avista Capital, Inc. (Avista Capital), a wholly owned non-regulated subsidiary of Avista Corp., is the parent company of all of the subsidiary companies in the non-utility businesses, with the exception of AJT Mining Properties, Inc., which is a subsidiary of AERC. On July 19, 2017, Avista Corp. entered into an Agreement and Plan of Merger (Merger Agreement) to become a wholly-owned subsidiary of Hydro One Limited (Hydro One). Consummation of the pending acquisition is subject to a number of approvals and the satisfaction or waiver of other specified conditions. The transaction is expected to close in the second half of 2018. See Note 13 for additional information. Basis of Reporting The condensed consolidated financial statements include the assets, liabilities, revenues and expenses of the Company and its subsidiaries and other majority owned subsidiaries and variable interest entities for which the Company or its subsidiaries are the primary beneficiaries. Intercompany balances were eliminated in consolidation. The accompanying condensed consolidated financial statements include the Company’s proportionate share of utility plant and related operations resulting from its interests in jointly owned plants. Taxes Other Than Income Taxes Taxes other than income taxes include state excise taxes, city occupational and franchise taxes, real and personal property taxes and certain other taxes not based on income. These taxes are generally based on revenues or the value of property. Utility-related taxes collected from customers (primarily state excise taxes and city utility taxes) are recorded as operating revenue and expense. Taxes other than income taxes consisted of the following items for the three and nine months ended September 30 (dollars in thousands): Three months ended September 30, Nine months ended September 30, 2017 2016 2017 2016 Utility-related taxes $ 12,663 $ 12,095 $ 47,799 $ 43,033 Property taxes 9,991 10,047 29,829 29,757 Other taxes 615 527 2,105 1,879 Total $ 23,269 $ 22,669 $ 79,733 $ 74,669 Materials and Supplies, Fuel Stock and Stored Natural Gas Inventories of materials and supplies, fuel stock and stored natural gas are recorded at average cost for our regulated operations and the lower of cost or net realizable value for our non-regulated operations and consisted of the following as of September 30, 2017 and December 31, 2016 (dollars in thousands): September 30, December 31, 2017 2016 Materials and supplies $ 41,518 $ 40,700 Fuel stock 5,064 4,585 Stored natural gas 19,379 8,029 Total $ 65,961 $ 53,314 Derivative Assets and Liabilities Derivatives are recorded as either assets or liabilities on the Condensed Consolidated Balance Sheets measured at estimated fair value. The Washington Utilities and Transportation Commission (UTC) and the Idaho Public Utilities Commission (IPUC) issued accounting orders authorizing Avista Corp. to offset energy commodity derivative assets or liabilities with a regulatory asset or liability. This accounting treatment is intended to defer the recognition of mark-to-market gains and losses on energy commodity transactions until the period of delivery. Realized benefits and costs result in adjustments to retail rates through purchased gas cost adjustments, the Energy Recovery Mechanism (ERM) in Washington, the Power Cost Adjustment (PCA) mechanism in Idaho, and periodic general rate cases. The resulting regulatory assets have been concluded to be probable of recovery through future rates. Substantially all forward contracts to purchase or sell power and natural gas are recorded as derivative assets or liabilities at estimated fair value with an offsetting regulatory asset or liability. Contracts that are not considered derivatives are accounted for on the accrual basis until they are settled or realized unless there is a decline in the fair value of the contract that is determined to be other-than-temporary. For interest rate swap derivatives, Avista Corp. records all mark-to-market gains and losses in each accounting period as assets and liabilities, as well as offsetting regulatory assets and liabilities, such that there is no income statement impact. The interest rate swap derivatives are risk management tools similar to energy commodity derivatives. Upon settlement of interest rate swap derivatives, the regulatory asset or liability is amortized as a component of interest expense over the term of the associated debt. The Company records an offset of interest rate swap derivative assets and liabilities with regulatory assets and liabilities, based on the prior practice of the commissions to provide recovery through the ratemaking process. As of September 30, 2017 , the Company has multiple master netting agreements with a variety of entities that allow for cross-commodity netting of derivative agreements with the same counterparty (i.e. power derivatives can be netted with natural gas derivatives). In addition, some master netting agreements allow for the netting of commodity derivatives and interest rate swap derivatives for the same counterparty. The Company does not have any agreements which allow for cross-affiliate netting among multiple affiliated legal entities. The Company nets all derivative instruments when allowed by the agreement for presentation in the Condensed Consolidated Balance Sheets. Fair Value Measurements Fair value represents the price that would be received when selling an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the measurement date. Energy commodity derivative assets and liabilities, deferred compensation assets, as well as derivatives related to interest rate swaps and foreign currency exchange contracts, are reported at estimated fair value on the Condensed Consolidated Balance Sheets. See Note 8 for the Company’s fair value disclosures. Accumulated Other Comprehensive Loss Accumulated other comprehensive loss, net of tax, consisted of the following as of September 30, 2017 and December 31, 2016 (dollars in thousands): September 30, December 31, 2017 2016 Unfunded benefit obligation for pensions and other postretirement benefit plans - net of taxes of $3,780 and $4,075, respectively $ 7,020 $ 7,568 The following table details the reclassifications out of accumulated other comprehensive loss by component for the three and nine months ended September 30 (dollars in thousands). Amounts Reclassified from Accumulated Other Comprehensive Loss Three months ended September 30, Nine months ended September 30, Details about Accumulated Other Comprehensive Loss Components 2017 2016 2017 2016 Affected Line Item in Statement of Income Amortization of defined benefit pension items Amortization of net prior service cost $ (300 ) $ (312 ) $ (898 ) $ (934 ) (a) Amortization of net loss 3,637 3,642 $ 10,913 $ 10,926 (a) Adjustment due to effects of regulation (3,057 ) (3,115 ) (9,172 ) (11,453 ) (a) (b) 280 215 843 (1,461 ) Total before tax (98 ) (75 ) (295 ) 512 Tax benefit (expense) $ 182 $ 140 $ 548 $ (949 ) Net of tax (a) These accumulated other comprehensive loss components are included in the computation of net periodic pension cost (see Note 4 for additional details). (b) The adjustment for the effects of regulation during the nine months ended September 30, 2016 includes approximately $2.1 million related to the reclassification of a pension regulatory asset associated with one of our jurisdictions into accumulated other comprehensive loss. Effective Income Tax Rate For the three months ended September 30, 2017 and 2016 , the Company's effective tax rate was 53.6 percent and 38.3 percent , respectively. For the nine months ended September 30, 2017 and 2016 , the Company's effective tax rate was 36.9 percent and 36.0 percent , respectively. The effective tax rate increased during 2017 because the majority of acquisition costs, which reduce income before income taxes, are not deductible for tax purposes and thus do not reduce income tax expense. Contingencies The Company has unresolved regulatory, legal and tax issues which have inherently uncertain outcomes. The Company accrues a loss contingency if it is probable that a liability has been incurred and the amount of the loss or impairment can be reasonably estimated. The Company also discloses loss contingencies that do not meet these conditions for accrual if there is a reasonable possibility that a material loss may be incurred. As of September 30, 2017 , the Company has not recorded any significant amounts related to unresolved contingencies. See Note 11 for further discussion of the Company's commitments and contingencies.

New Accounting Standards

New Accounting Standards	9 Months Ended
	Sep. 30, 2017
New Accounting Pronouncements and Changes in Accounting Principles [Abstract]
New Accounting Standards	NEW ACCOUNTING STANDARDS ASU No. 2014-09, “Revenue from Contracts with Customers (Topic 606)” In May 2014, the FASB issued ASU No. 2014-09, which outlines a single comprehensive model for entities to use in accounting for revenue arising from contracts with customers and supersedes most current revenue recognition guidance, including industry-specific guidance. The core principle of the revenue model is that an entity should identify the various performance obligations in a contract, allocate the transaction price among the performance obligations and recognize revenue when (or as) the entity satisfies each performance obligation. This ASU is effective for periods beginning after December 15, 2017. The Company has a revenue recognition standard implementation team that is working through the implementation process. The Company has evaluated this standard and is planning to adopt this standard in 2018 upon its effective date. The Company is expecting to use a modified retrospective method of adoption, which would require a cumulative adjustment to opening retained earnings, as opposed to a full retrospective application. Based on work performed to date, the Company has not identified any material cumulative adjustments necessary. Since the majority of Avista Corp.’s revenue is from rate-regulated sales of electricity and natural gas to retail customers and revenue is recognized as energy is delivered to these customers, the Company does not expect a significant change in operating revenues or net income. The Company has reviewed and analyzed certain contracts with customers (most of which are related to wholesale sales of power and natural gas) and has not yet identified any significant differences in revenue recognition between current GAAP and ASU No. 2014-09. During the implementation process, the Company has identified several issues, the most significant of which are as follows based on our current assessment: Contributions in Aid of Construction – There was the potential that contributions in aid of construction (CIACs) could be recognized as revenue upon the adoption of ASU No. 2014-09. Under current GAAP, CIACs are accounted for as an offset to the cost of utility plant in service. Current implementation guidance indicates that CIACs will continue to be accounted for as an offset to utility plant in service. Utility-Related Taxes Collected from Customers – There were questions on the presentation of utility-related taxes collected from customers (primarily state excise taxes and city utility taxes) on a gross basis. Under current GAAP, the Company is allowed to record these utility-related taxes on a gross basis in revenue when billed to customers with an offset included in taxes other than income taxes in operating expenses. The Company evaluated whether this gross presentation is appropriate under ASU 2014-09 and the Company's assessment indicates that there will be no material changes to current presentation. Renewable Energy Credits - Current utility industry implementation guidance indicates that revenue associated with the sale of self-generated renewable energy credits (REC) will be recognized at the time of generation and sale of the credits as opposed to when the RECs are certified in the Western Renewable Energy Generation Information System (WREGIS), which generally occurs during a period subsequent to the sale. This represents a change from the Company's current practice, which is to defer revenue recognition until the time of certification. Revenue associated with the sale of RECs is not material to the financial statements; therefore, this change will not materially impact the amount of revenue recognized each period. Additionally, almost all of the Company's REC revenue is deferred for future rebate to retail customers. As such, any change in the timing of revenue recognition would not have any material impact on net income. The Company is monitoring utility industry implementation guidance as it relates to the above issues to determine if there will be a final industry consensus regarding accounting and presentation of these items. In addition to the issues described above, the Company also expects significant changes to its revenue-related footnote disclosures, including the bifurcation of wholesale revenue into derivative and non-derivative sales. The Company continues to evaluate what information would be most useful for users of the financial statements, including information already provided elsewhere in the document outside the footnote disclosures. These additional disclosures could include the disaggregation of revenues by type of service, source of revenue or customer class. Also, the Company expects enhanced disclosures regarding its revenue recognition policies and elections. ASU No. 2016-02 “Leases (Topic 842)” In February 2016, the FASB issued ASU No. 2016-02. This ASU introduces a new lessee model that requires most leases to be capitalized and shown on the balance sheet with corresponding lease assets and liabilities. The standard also aligns certain of the underlying principles of the new lessor model with those in Topic 606, the FASB’s new revenue recognition standard. Furthermore, this ASU addresses other issues that arise under the current lease model; for example, eliminating the required use of bright-line tests in current GAAP for determining lease classification (operating leases versus capital leases). This ASU also includes enhanced disclosures surrounding leases. This ASU is effective for periods beginning on or after December 15, 2018; however, early adoption is permitted. Upon adoption, this ASU must be applied using a modified retrospective approach to the earliest period presented, which will likely require restatements of previously issued financial statements. The modified retrospective approach includes a number of optional practical expedients that entities may elect to apply. The Company evaluated this standard and determined that it will most likely not early adopt this standard before its effective date in 2019. The Company has formed a lease standard implementation team that is working through the implementation process. Based on work to-date, the implementation team has identified a complete population of existing and potential leases under the new standard and has completed its review of the agreements associated with this population. However, the team has not yet quantified the impact of recording these leases. In addition, the team is developing a process to identify any new potential leases that may be entered into between now and the standard implementation date in 2019. The Company is monitoring utility industry implementation guidance as it relates to several unresolved issues to determine if there will be an industry consensus, including whether right-of-ways are considered leases. The Company has not yet estimated the potential impact on its future financial condition, results of operations and cash flows. ASU No. 2016-09 “Compensation—Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting” In March 2016, the FASB issued ASU No. 2016-09. This ASU simplified several aspects of the accounting for employee share-based payment transactions including: • allowing excess tax benefits or tax deficiencies to be recognized as income tax benefits or expenses in the Condensed Consolidated Statements of Income rather than in Additional Paid in Capital (APIC), • excess tax benefits no longer represent a financing cash inflow on the Condensed Consolidated Statements of Cash Flows and instead will be included as an operating activity, • requiring excess tax benefits and tax deficiencies to be excluded from the calculation of diluted earnings per share, whereas under previous accounting guidance, these amounts had to be estimated and included in the calculation, • allowing forfeitures to be accounted for as they occur, instead of estimating forfeitures, and • changing the statutory tax withholding requirements for share-based payments. The Company early adopted this standard during the second quarter of 2016, with a retrospective effective date of January 1, 2016. The adoption of this standard resulted in a recognized income tax benefit of $1.6 million in 2016 associated with excess tax benefits on settled share-based employee payments. ASU No. 2017-07 “Compensation-Retirement Benefits (Topic 715): Improving the Presentation of Net Periodic Pension Cost and Net Periodic Postretirement Benefit Cost” In March 2017, the FASB issued ASU No. 2017-07, which amends the income statement presentation of the components of net period benefit cost for an entity’s defined benefit pension and other postretirement plans. Under current GAAP, net benefit cost consists of several components that reflect different aspects of an employer’s financial arrangements as well as the cost of benefits earned by employees. These components are aggregated and reported net in the financial statements. ASU No. 2017-07 requires entities to (1) disaggregate the current service-cost component from the other components of net benefit cost (other components) and present it with other current compensation costs for related employees in the income statement and (2) present the other components elsewhere in the income statement and outside of income from operations. In addition, only the service-cost component of net benefit cost is eligible for capitalization (e.g., as part of utility plant). This is a change from current practice, under which entities capitalize the aggregate net benefit cost to utility plant when applicable, in accordance with Federal Energy and Regulatory Commission (FERC) accounting guidance. Avista Corp. is a rate-regulated entity and all components of net benefit cost are currently recovered from rate payers as a component of utility plant and, under the new ASU, these costs will continue to be recovered from rate payers in the same manner over the depreciable lives of utility plant. As all such costs are expected to continue to be recoverable, the components that are no longer eligible to be recorded as a component of plant for GAAP will be recorded as regulatory assets. This ASU is effective for periods beginning after December 15, 2017 and early adoption is permitted. Upon adoption, entities must use a retrospective transition method to adopt the requirement for separate presentation in the income statement and a prospective transition method to adopt the requirement to limit the capitalization of benefit costs to the service-cost component. The Company did not early adopt this standard and does not expect a material impact on its future financial condition, results of operations or cash flows upon adoption of this standard.

Derivatives And Risk Management

Derivatives And Risk Management	9 Months Ended
	Sep. 30, 2017
Derivative Instruments and Hedges, Assets [Abstract]
Derivatives And Risk Management	DERIVATIVES AND RISK MANAGEMENT The disclosures below in Note 3 apply only to Avista Corp. and its operating division Avista Utilities; AERC and its primary subsidiary AEL&P do not enter into derivative instruments. Energy Commodity Derivatives Avista Corp. is exposed to market risks relating to changes in electricity and natural gas commodity prices and certain other fuel prices. Market risk is, in general, the risk of fluctuation in the market price of the commodity being traded and is influenced primarily by supply and demand. Market risk includes the fluctuation in the market price of associated derivative commodity instruments. Avista Corp. utilizes derivative instruments, such as forwards, futures, swap derivatives and options in order to manage the various risks relating to these commodity price exposures. Avista Corp. has an energy resources risk policy and control procedures to manage these risks. As part of Avista Corp.'s resource procurement and management operations in the electric business, Avista Corp. engages in an ongoing process of resource optimization, which involves the economic selection from available energy resources to serve Avista Corp.'s load obligations and the use of these resources to capture available economic value. Avista Corp. transacts in wholesale markets by selling and purchasing electric capacity and energy, fuel for electric generation, and derivative contracts related to capacity, energy and fuel. Such transactions are part of the process of matching resources with load obligations and hedging a portion of the related financial risks. These transactions range from terms of intra-hour up to multiple years. As part of its resource procurement and management of its natural gas business, Avista Corp. makes continuing projections of its natural gas loads and assesses available natural gas resources including natural gas storage availability. Natural gas resource planning typically includes peak requirements, low and average monthly requirements and delivery constraints from natural gas supply locations to Avista Corp.’s distribution system. However, daily variations in natural gas demand can be significantly different than monthly demand projections. On the basis of these projections, Avista Corp. plans and executes a series of transactions to hedge a portion of its projected natural gas requirements through forward market transactions and derivative instruments. These transactions may extend as much as four natural gas operating years (November through October) into the future. Avista Corp. also leaves a significant portion of its natural gas supply requirements unhedged for purchase in short-term and spot markets. Avista Corp. plans for sufficient natural gas delivery capacity to serve its retail customers for a theoretical peak day event. Avista Corp. generally has more pipeline and storage capacity than what is needed during periods other than a peak-day. Avista Corp. optimizes its natural gas resources by using market opportunities to generate economic value that helps mitigate fixed costs. Avista Corp. also optimizes its natural gas storage capacity by purchasing and storing natural gas when prices are traditionally lower, typically in the summer, and withdrawing during higher priced months, typically during the winter. However, if market conditions and prices indicate that Avista Corp. should buy or sell natural gas at other times during the year, Avista Corp. engages in optimization transactions to capture value in the marketplace. Natural gas optimization activities include, but are not limited to, wholesale market sales of surplus natural gas supplies, purchases and sales of natural gas to optimize use of pipeline and storage capacity, and participation in the transportation capacity release market. The following table presents the underlying energy commodity derivative volumes as of September 30, 2017 that are expected to be delivered in each respective year (in thousands of MWhs and mmBTUs): Purchases Sales Electric Derivatives Gas Derivatives Electric Derivatives Gas Derivatives Year Physical (1) MWH Financial (1) MWH Physical (1) mmBTUs Financial (1) mmBTUs Physical (1) Financial (1) Physical (1) Financial (1) Remainder 2017 191 303 5,018 33,165 160 367 1,519 17,878 2018 418 763 3,040 90,750 223 1,275 2,177 63,625 2019 235 737 610 44,290 158 982 1,345 30,050 2020 — — 910 7,750 — 246 1,430 — 2021 — — — — — — 1,049 — Thereafter — — — — — — — — The following table presents the underlying energy commodity derivative volumes as of December 31, 2016 that are expected to be delivered in each respective year (in thousands of MWhs and mmBTUs): Purchases Sales Electric Derivatives Gas Derivatives Electric Derivatives Gas Derivatives Year Physical (1) MWH Financial (1) MWH Physical (1) mmBTUs Financial (1) mmBTUs Physical (1) Financial (1) Physical (1) Financial (1) 2017 510 907 15,475 110,380 316 1,552 4,165 73,110 2018 397 — — 52,755 286 1,244 1,360 15,113 2019 235 — 610 29,475 158 982 1,345 4,020 2020 — — 910 2,725 — — 1,430 — 2021 — — — — — — 1,060 — Thereafter — — — — — — — — (1) Physical transactions represent commodity transactions in which Avista Corp. will take or make delivery of either electricity or natural gas; financial transactions represent derivative instruments with delivery of cash in the amount of the benefit or cost but with no physical delivery of the commodity, such as futures, swap derivatives, options, or forward contracts. The electric and natural gas derivative contracts above will be included in either power supply costs or natural gas supply costs during the period they are delivered and will be included in the various recovery mechanisms (ERM, PCA, and Purchased Gas Adjustments (PGA)), or in the general rate case process, and are expected to be collected through retail rates from customers. Foreign Currency Exchange Derivatives A significant portion of Avista Corp.’s natural gas supply (including fuel for power generation) is obtained from Canadian sources. Most of those transactions are executed in U.S. dollars, which avoids foreign currency risk. A portion of Avista Corp.’s short-term natural gas transactions and long-term Canadian transportation contracts are committed based on Canadian currency prices and settled within 60 days with U.S. dollars. Avista Corp. hedges a portion of the foreign currency risk by purchasing Canadian currency exchange derivatives when such commodity transactions are initiated. The foreign currency exchange derivatives and the unhedged foreign currency risk have not had a material effect on Avista Corp.’s financial condition, results of operations or cash flows and these differences in cost related to currency fluctuations are included with natural gas supply costs for ratemaking. The following table summarizes the foreign currency exchange derivatives that Avista Corp. has outstanding as of September 30, 2017 and December 31, 2016 (dollars in thousands): September 30, December 31, 2017 2016 Number of contracts 19 21 Notional amount (in United States dollars) $ 3,198 $ 2,819 Notional amount (in Canadian dollars) 3,925 3,754 Interest Rate Derivatives Avista Corp. is affected by fluctuating interest rates related to a portion of its existing debt, and future borrowing requirements. Avista Corp. hedges a portion of its interest rate risk with financial derivative instruments, which may include interest rate swap derivatives and U.S. Treasury lock agreements. These interest rate swap derivatives and U.S. Treasury lock agreements are considered economic hedges against fluctuations in future cash flows associated with anticipated debt issuances. The following table summarizes the unsettled interest rate swap derivatives that Avista Corp. has outstanding as of September 30, 2017 and December 31, 2016 (dollars in thousands): Balance Sheet Date Number of Contracts Notional Amount Mandatory Cash Settlement Date September 30, 2017 14 $ 275,000 2018 6 70,000 2019 3 30,000 2020 1 15,000 2021 5 60,000 2022 December 31, 2016 6 $ 75,000 2017 14 275,000 2018 6 70,000 2019 2 20,000 2020 5 60,000 2022 During the third quarter 2017, in connection with the pricing of $90.0 million of Avista Corp. first mortgage bonds that are expected to be issued in December 2017 (see Note 6), the Company cash-settled five interest rate swap derivatives (notional aggregate amount of $60.0 million ) and paid a net amount of $8.8 million . Upon settlement of interest rate swap derivatives, the cash payments made or received are recorded as a regulatory asset or liability and are subsequently amortized as a component of interest expense over the life of the associated debt. The settled interest rate swap derivatives are also included as a part of the Company's cost of debt calculation for ratemaking purposes. The fair value of outstanding interest rate swap derivatives can vary significantly from period to period depending on the total notional amount of swap derivatives outstanding and fluctuations in market interest rates compared to the interest rates fixed by the swaps. Avista Corp. is required to make cash payments to settle the interest rate swap derivatives when the fixed rates are higher than prevailing market rates at the date of settlement. Conversely, Avista Corp. receives cash to settle its interest rate swap derivatives when prevailing market rates at the time of settlement exceed the fixed swap rates. Upon settlement of interest rate swaps, the cash payments made or received are recorded as a regulatory asset or liability and are amortized as a component of interest expense over the life of the associated debt. The settled interest rate swaps are also included as a part of the Company's cost of debt calculation for ratemaking purposes. Summary of Outstanding Derivative Instruments The amounts recorded on the Condensed Consolidated Balance Sheet as of September 30, 2017 and December 31, 2016 reflect the offsetting of derivative assets and liabilities where a legal right of offset exists. The following table presents the fair values and locations of derivative instruments recorded on the Condensed Consolidated Balance Sheet as of September 30, 2017 (in thousands): Fair Value as of September 30, 2017 Derivative and Balance Sheet Location Gross Asset Gross Liability Collateral Netted Net Asset (Liability) on Balance Sheet Foreign currency exchange derivatives Other current liabilities $ — $ (51 ) $ — $ (51 ) Interest rate swap derivatives Other current assets 2,637 (249 ) — 2,388 Other property and investments-net and other non-current assets 5,346 (1,896 ) — 3,450 Current interest rate swap derivative liabilities — (63,074 ) 28,554 (34,520 ) Non-current interest rate swap derivative liabilities — (7,136 ) 5,806 (1,330 ) Energy commodity derivatives Other current assets 197 (70 ) — 127 Current energy commodity derivative liabilities 23,904 (46,036 ) 11,078 (11,054 ) Other non-current liabilities, regulatory liabilities and deferred credits 13,891 (30,262 ) 4,569 (11,802 ) Total derivative instruments recorded on the balance sheet $ 45,975 $ (148,774 ) $ 50,007 $ (52,792 ) The following table presents the fair values and locations of derivative instruments recorded on the Condensed Consolidated Balance Sheet as of December 31, 2016 (in thousands): Fair Value as of December 31, 2016 Derivative and Balance Sheet Location Gross Asset Gross Liability Collateral Net Asset Foreign currency exchange derivatives Other current liabilities $ 5 $ (28 ) $ — $ (23 ) Interest rate swap derivatives Other current assets 3,393 — — 3,393 Other property and investments-net and other non-current assets 5,754 (397 ) — 5,357 Current interest rate swap derivative liabilities — (15,756 ) 9,731 (6,025 ) Non-current interest rate swap derivative liabilities 3,951 (57,825 ) 25,169 (28,705 ) Energy commodity derivatives Other current assets 18,682 (16,787 ) — 1,895 Current energy commodity derivative liabilities 16,335 (29,598 ) 6,228 (7,035 ) Other non-current liabilities, regulatory liabilities and deferred credits 13,071 (29,990 ) 3,630 (13,289 ) Total derivative instruments recorded on the balance sheet $ 61,191 $ (150,381 ) $ 44,758 $ (44,432 ) Exposure to Demands for Collateral Avista Corp.'s derivative contracts often require collateral (in the form of cash or letters of credit) or other credit enhancements, or reductions or terminations of a portion of the contract through cash settlement. In the event of a downgrade in Avista Corp.'s credit ratings or changes in market prices, additional collateral may be required. In periods of price volatility, the level of exposure can change significantly. As a result, sudden and significant demands may be made against Avista Corp.'s credit facilities and cash. Avista Corp. actively monitors the exposure to possible collateral calls and takes steps to mitigate capital requirements. The following table presents Avista Corp.'s collateral outstanding related to its derivative instruments as of September 30, 2017 and December 31, 2016 (in thousands): September 30, December 31, 2017 2016 Energy commodity derivatives Cash collateral posted $ 19,570 $ 17,134 Letters of credit outstanding 31,500 24,400 Balance sheet offsetting (cash collateral against net derivative positions) 15,647 9,858 Interest rate swap derivatives Cash collateral posted 34,360 34,900 Letters of credit outstanding 6,000 3,600 Balance sheet offsetting (cash collateral against net derivative positions) 34,360 34,900 Certain of Avista Corp.’s derivative instruments contain provisions that require Avista Corp. to maintain an "investment grade" credit rating from the major credit rating agencies. If Avista Corp.’s credit ratings were to fall below "investment grade," it would be in violation of these provisions, and the counterparties to the derivative instruments could request immediate payment or demand immediate and ongoing collateralization on derivative instruments in net liability positions. The following table presents the aggregate fair value of all derivative instruments with credit-risk-related contingent features that are in a liability position and the amount of additional collateral Avista Corp. could be required to post as of September 30, 2017 and December 31, 2016 (in thousands): September 30, December 31, 2017 2016 Energy commodity derivatives Liabilities with credit-risk-related contingent features $ 670 $ 1,124 Additional collateral to post 670 1,046 Interest rate swap derivatives Liabilities with credit-risk-related contingent features 72,355 73,978 Additional collateral to post 12,730 21,100

Pension Plans And Other Postret

Pension Plans And Other Postretirement Benefit Plans	9 Months Ended
	Sep. 30, 2017
Retirement Benefits, Description [Abstract]
Pension Plans and Other Postretirement Benefit Plans	PENSION PLANS AND OTHER POSTRETIREMENT BENEFIT PLANS Avista Utilities Avista Utilities’ pension and other postretirement plans have not changed during the nine months ended September 30, 2017 . The Company’s funding policy is to contribute at least the minimum amounts that are required to be funded under the Employee Retirement Income Security Act, but not more than the maximum amounts that are currently deductible for income tax purposes. The Company contributed $22.0 million in cash to the pension plan for the nine months ended September 30, 2017 and does not expect any further contributions during 2017. The Company contributed $12.0 million in cash to the pension plan in 2016 . The Company uses a December 31 measurement date for its defined benefit pension and other postretirement benefit plans. The following table sets forth the components of net periodic benefit costs for the three and nine months ended September 30 (dollars in thousands): Pension Benefits Other Post-retirement Benefits 2017 2016 2017 2016 Three months ended September 30: Service cost $ 5,092 $ 4,567 $ 799 $ 806 Interest cost 6,976 6,895 1,374 1,530 Expected return on plan assets (7,900 ) (6,887 ) (475 ) (465 ) Amortization of prior service cost — 1 (274 ) (300 ) Net loss recognition 2,517 2,161 1,168 1,453 Net periodic benefit cost $ 6,685 $ 6,737 $ 2,592 $ 3,024 Nine months ended September 30: Service cost $ 15,226 $ 13,655 $ 2,422 $ 2,389 Interest cost 20,903 20,695 4,147 4,623 Expected return on plan assets (23,700 ) (20,512 ) (1,425 ) (1,415 ) Amortization of prior service cost — 1 (898 ) (924 ) Net loss recognition 7,380 6,252 3,761 4,312 Net periodic benefit cost $ 19,809 $ 20,091 $ 8,007 $ 8,985 Total net periodic benefit costs in the table above are recorded to the same accounts as labor expense. Labor and benefits expense is recorded to various projects based on whether the work is a capital project or an operating expense. Approximately 40 percent of all labor and benefits is capitalized to utility property and 60 percent is expensed to other operating expenses.

Committed Lines of Credit

Committed Lines of Credit	9 Months Ended
	Sep. 30, 2017
Short-term Debt [Abstract]
Committed Lines of Credit	COMMITTED LINES OF CREDIT Avista Corp. Avista Corp. has a committed line of credit with various financial institutions in the total amount of $400.0 million that expires in April 2021 . The committed line of credit is secured by non-transferable first mortgage bonds of the Company issued to the agent bank that would only become due and payable in the event, and then only to the extent, that the Company defaults on its obligations under the committed line of credit. Borrowings outstanding and interest rates of borrowings (excluding letters of credit) under the Company’s revolving committed line of credit were as follows as of September 30, 2017 and December 31, 2016 (dollars in thousands): September 30, December 31, 2017 2016 Borrowings outstanding at end of period (1) $ 195,000 $ 120,000 Letters of credit outstanding at end of period $ 43,853 $ 34,353 Average interest rates at end of period 1.99 % 1.50 % (1) As of September 30, 2017 , there was $195.0 million outstanding under the committed line of credit; however, $105.9 million was classified as short-term borrowings and the remaining $89.1 million was classified as long-term debt on the Condensed Consolidated Balance Sheet due to the Company's intention to refinance such amount on a long-term basis through the issuance and sale of first mortgage bonds in December 2017 pursuant to a bond purchase agreement entered into in September 2017. See Note 7 for further discussion of the bond purchase agreement and the refinancing of short-term debt on a long-term basis. In addition, there were short-term borrowings outstanding as of September 30, 2017 on the Condensed Consolidated Balance Sheet related to a short-term note payable by a subsidiary for the acquisition of land that is expected to be repaid in early 2018. AEL&P AEL&P has a committed line of credit in the amount of $25.0 million that expires in November 2019 . As of September 30, 2017 and December 31, 2016 , there were no borrowings or letters of credit outstanding under this committed line of credit. The committed line of credit is secured by non-transferable first mortgage bonds of AEL&P issued to the agent bank that would only become due and payable in the event, and then only to the extent, that AEL&P defaults on its obligations under the committed line of credit.

Long-Term Debt and Capital Leas

Long-Term Debt and Capital Leases Long-Term Debt and Capital Leases (Notes)	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Debt and Capital Leases Disclosures [Text Block]	LONG-TERM DEBT AND CAPITAL LEASES The following details long-term debt outstanding as of September 30, 2017 and December 31, 2016 (dollars in thousands): Maturity Year Description Interest Rate September 30, December 31, Avista Corp. Secured Long-Term Debt 2018 First Mortgage Bonds 5.95% $ 250,000 $ 250,000 2018 Secured Medium-Term Notes 7.39%-7.45% 22,500 22,500 2019 First Mortgage Bonds 5.45% 90,000 90,000 2020 First Mortgage Bonds 3.89% 52,000 52,000 2022 First Mortgage Bonds 5.13% 250,000 250,000 2023 Secured Medium-Term Notes 7.18%-7.54% 13,500 13,500 2028 Secured Medium-Term Notes 6.37% 25,000 25,000 2032 Secured Pollution Control Bonds (1) (1) 66,700 66,700 2034 Secured Pollution Control Bonds (1) (1) 17,000 17,000 2035 First Mortgage Bonds 6.25% 150,000 150,000 2037 First Mortgage Bonds 5.70% 150,000 150,000 2040 First Mortgage Bonds 5.55% 35,000 35,000 2041 First Mortgage Bonds 4.45% 85,000 85,000 2044 First Mortgage Bonds 4.11% 60,000 60,000 2045 First Mortgage Bonds 4.37% 100,000 100,000 2047 First Mortgage Bonds 4.23% 80,000 80,000 2051 First Mortgage Bonds 3.54% 175,000 175,000 Total Avista Corp. secured long-term debt 1,621,700 1,621,700 Alaska Electric Light and Power Company Secured Long-Term Debt 2044 First Mortgage Bonds 4.54% 75,000 75,000 Total secured long-term debt 1,696,700 1,696,700 Alaska Energy and Resources Company Unsecured Long-Term Debt 2019 Unsecured Term Loan 3.85% 15,000 15,000 Total secured and unsecured long-term debt 1,711,700 1,711,700 Other Long-Term Debt Components Capital lease obligations 62,969 65,435 Unamortized debt discount (668 ) (792 ) Unamortized long-term debt issuance costs (9,986 ) (10,639 ) Committed line of credit to be refinanced on a long-term basis (2) 89,100 — Total 1,853,115 1,765,704 Secured Pollution Control Bonds held by Avista Corporation (1) (83,700 ) (83,700 ) Current portion of long-term debt and capital leases (277,626 ) (3,287 ) Total long-term debt and capital leases $ 1,491,789 $ 1,678,717 (1) In December 2010, $66.7 million and $17.0 million of the City of Forsyth, Montana Pollution Control Revenue Refunding Bonds (Avista Corporation Colstrip Project) due in 2032 and 2034 , respectively, which had been held by Avista Corp. since 2008 and 2009, respectively, were refunded by new variable rate bond issues (Series 2010A and Series 2010B). The new bonds were not offered to the public and were purchased by Avista Corp. due to market conditions. The Company expects that at a later date, subject to market conditions, these bonds may be remarketed to unaffiliated investors. So long as Avista Corp. is the holder of these bonds, the bonds will not be reflected as an asset or a liability on Avista Corp.'s Consolidated Balance Sheets. (2) In September 2017, the Company entered into a bond purchase agreement with four institutional investors in the private placement market for the issuance and sale of $90.0 million of Avista Corp. first mortgage bonds in December 2017. The first mortgage bonds will bear a coupon rate of 3.91 percent and mature in December 2047 . The Company intends to use the $90.0 million of bond proceeds, less issuance costs, to refinance on a long-term basis $89.1 million outstanding under the Company's committed line of credit. As such, $89.1 million has been excluded from current liabilities and is recorded as long-term debt on the Condensed Consolidated Balance Sheets as of September 30, 2017 . In connection with the pricing of the first mortgage bonds, the Company cash-settled five interest rate swap derivatives (notional aggregate amount of $60.0 million ) and paid a net amount of $8.8 million .

Long- Term Debt to Affiliated T

Long- Term Debt to Affiliated Trust Long-Term Debt to Affiliated Trust (Notes)	9 Months Ended
	Sep. 30, 2017
Long-Term Debt to Affiliated Trust [Abstract]
Long Term Debt To Affiliated Trusts Disclosure [Text Block]	LONG-TERM DEBT TO AFFILIATED TRUSTS In 1997, the Company issued Floating Rate Junior Subordinated Deferrable Interest Debentures, Series B, with a principal amount of $51.5 million to Avista Capital II, an affiliated business trust formed by the Company. Avista Capital II issued $50.0 million of Preferred Trust Securities with a floating distribution rate of LIBOR plus 0.875 percent , calculated and reset quarterly. The distribution rates paid were as follows during the nine months ended September 30, 2017 and the year ended December 31, 2016 : September 30, December 31, 2017 2016 Low distribution rate 1.81 % 1.29 % High distribution rate 2.19 % 1.81 % Distribution rate at the end of the period 2.19 % 1.81 % Concurrent with the issuance of the Preferred Trust Securities, Avista Capital II issued $1.5 million of Common Trust Securities to the Company. These debt securities may be redeemed at the option of Avista Capital II at any time and mature on June 1, 2037. In December 2000, the Company purchased $10.0 million of these Preferred Trust Securities. The Company owns 100 percent of Avista Capital II and has solely and unconditionally guaranteed the payment of distributions on, and redemption price and liquidation amount for, the Preferred Trust Securities to the extent that Avista Capital II has funds available for such payments from the respective debt securities. Upon maturity or prior redemption of such debt securities, the Preferred Trust Securities will be mandatorily redeemed. The Company does not include these capital trusts in its consolidated financial statements as Avista Corp. is not the primary beneficiary. As such, the sole assets of the capital trusts are $51.5 million of junior subordinated deferrable interest debentures of Avista Corp., which are reflected on the Condensed Consolidated Balance Sheets. Interest expense to affiliated trusts in the Condensed Consolidated Statements of Income represents interest expense on these debentures.

Fair Value

Fair Value	9 Months Ended
	Sep. 30, 2017
Fair Value Disclosures [Abstract]
Fair Value	FAIR VALUE The carrying values of cash and cash equivalents, accounts and notes receivable, accounts payable, and short-term borrowings are reasonable estimates of their fair values. Long-term debt (including current portion and material capital leases) and long-term debt to affiliated trusts are reported at carrying value on the Condensed Consolidated Balance Sheets. The fair value hierarchy prioritizes the inputs used to measure fair value. The hierarchy gives the highest priority to unadjusted quoted prices in active markets for identical assets or liabilities (Level 1 measurement) and the lowest priority to fair values derived from unobservable inputs (Level 3 measurement). The three levels of the fair value hierarchy are defined as follows: Level 1 – Quoted prices are available in active markets for identical assets or liabilities. Active markets are those in which transactions for the asset or liability occur with sufficient frequency and volume to provide pricing information on an ongoing basis. Level 2 – Pricing inputs are other than quoted prices in active markets included in Level 1, but which are either directly or indirectly observable as of the reporting date. Level 2 includes those financial instruments that are valued using models or other valuation methodologies. These models are primarily industry-standard models that consider various assumptions, including quoted forward prices for commodities, time value, volatility factors, and current market and contractual prices for the underlying instruments, as well as other relevant economic measures. Substantially all of these assumptions are observable in the marketplace throughout the full term of the instrument, can be derived from observable data or are supported by observable levels at which transactions are executed in the marketplace. Level 3 – Pricing inputs include significant inputs that are generally unobservable from objective sources. These inputs may be used with internally developed methodologies that result in management’s best estimate of fair value. Financial assets and liabilities are classified in their entirety based on the lowest level of input that is significant to the fair value measurement. The Company’s assessment of the significance of a particular input to the fair value measurement requires judgment, and may affect the valuation of fair value assets and liabilities and their placement within the fair value hierarchy levels. The determination of the fair values incorporates various factors that not only include the credit standing of the counterparties involved and the impact of credit enhancements (such as cash deposits and letters of credit), but also the impact of Avista Corp.’s nonperformance risk on its liabilities. The following table sets forth the carrying value and estimated fair value of the Company’s financial instruments not reported at estimated fair value on the Condensed Consolidated Balance Sheets as of September 30, 2017 and December 31, 2016 (dollars in thousands): September 30, 2017 December 31, 2016 Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value Long-term debt (Level 2) $ 951,000 $ 1,072,557 $ 951,000 $ 1,048,661 Long-term debt (Level 3) 677,000 704,130 677,000 675,251 Snettisham capital lease obligation (Level 3) 60,349 62,000 62,160 62,800 Long-term debt to affiliated trusts (Level 3) 51,547 41,238 51,547 38,660 These estimates of fair value of long-term debt and long-term debt to affiliated trusts were primarily based on available market information, which generally consists of estimated market prices from third party brokers for debt with similar risk and terms. The price ranges obtained from the third party brokers consisted of par values of 80.00 to 129.64 , where a par value of 100.0 represents the carrying value recorded on the Condensed Consolidated Balance Sheets. Level 2 long-term debt represents publicly issued bonds with quoted market prices; however, due to their limited trading activity, they are classified as Level 2 because brokers must generate quotes and make estimates if there is no trading activity near a period end. Level 3 long-term debt consists of private placement bonds and debt to affiliated trusts, which typically have no secondary trading activity. Fair values in Level 3 are estimated based on market prices from third party brokers using secondary market quotes for debt with similar risk and terms to generate quotes for Avista Corp. bonds. Due to the unique nature of the Snettisham capital lease obligation, the estimated fair value of these items was determined based on a discounted cash flow model using available market information. The Snettisham capital lease obligation was discounted to present value using the Morgan Markets A Ex-Fin discount rate as published on September 30, 2017 . The following table discloses by level within the fair value hierarchy the Company’s assets and liabilities measured and reported on the Condensed Consolidated Balance Sheets as of September 30, 2017 and December 31, 2016 at fair value on a recurring basis (dollars in thousands): Level 1 Level 2 Level 3 Counterparty Total September 30, 2017 Assets: Energy commodity derivatives $ — $ 37,902 $ — $ (37,775 ) $ 127 Level 3 energy commodity derivatives: Natural gas exchange agreement — — 90 (90 ) — Interest rate swap derivatives — 7,983 — (2,145 ) 5,838 Deferred compensation assets: Fixed income securities (2) 1,692 — — — 1,692 Equity securities (2) 6,276 — — — 6,276 Total $ 7,968 $ 45,885 $ 90 $ (40,010 ) $ 13,933 Liabilities: Energy commodity derivatives $ — $ 56,013 $ — $ (53,422 ) $ 2,591 Level 3 energy commodity derivatives: Natural gas exchange agreement — — 3,689 (90 ) 3,599 Power exchange agreement — — 16,654 — 16,654 Power option agreement — — 12 — 12 Foreign currency exchange derivatives — 51 — — 51 Interest rate swap derivatives — 72,355 — (36,505 ) 35,850 Total $ — $ 128,419 $ 20,355 $ (90,017 ) $ 58,757 Level 1 Level 2 Level 3 Counterparty Total December 31, 2016 Assets: Energy commodity derivatives $ — $ 47,994 $ — $ (46,099 ) $ 1,895 Level 3 energy commodity derivatives: Natural gas exchange agreement — — 69 (69 ) — Power exchange agreement — — 25 (25 ) — Foreign currency exchange derivatives — 5 — (5 ) — Interest rate swap derivatives — 13,098 — (4,348 ) 8,750 Deferred compensation assets: Fixed income securities (2) 1,789 — — — 1,789 Equity securities (2) 5,481 — — — 5,481 Total $ 7,270 $ 61,097 $ 94 $ (50,546 ) $ 17,915 Liabilities: Energy commodity derivatives $ — $ 56,871 $ — $ (55,957 ) $ 914 Level 3 energy commodity derivatives: Natural gas exchange agreement — — 5,954 (69 ) 5,885 Power exchange agreement — — 13,474 (25 ) 13,449 Power option agreement — — 76 — 76 Foreign currency exchange derivatives — 28 — (5 ) 23 Interest rate swap derivatives — 73,978 — (39,248 ) 34,730 Total $ — $ 130,877 $ 19,504 $ (95,304 ) $ 55,077 (1) The Company is permitted to net derivative assets and derivative liabilities with the same counterparty when a legally enforceable master netting agreement exists. In addition, the Company nets derivative assets and derivative liabilities against any payables and receivables for cash collateral held or placed with these same counterparties. (2) These assets are trading securities and are included in other property and investments-net and other non-current assets on the Condensed Consolidated Balance Sheets. The difference between the amount of derivative assets and liabilities disclosed in respective levels in the table above and the amount of derivative assets and liabilities disclosed on the Condensed Consolidated Balance Sheets is due to netting arrangements with certain counterparties. See Note 3 for additional discussion of derivative netting. To establish fair value for energy commodity derivatives, the Company uses quoted market prices and forward price curves to estimate the fair value of energy commodity derivative instruments included in Level 2. In particular, electric derivative valuations are performed using market quotes, adjusted for periods in between quotable periods. Natural gas derivative valuations are estimated using New York Mercantile Exchange (NYMEX) pricing for similar instruments, adjusted for basin differences, using market quotes. Where observable inputs are available for substantially the full term of the contract, the derivative asset or liability is included in Level 2. To establish fair values for interest rate swap derivatives, the Company uses forward market curves for interest rates for the term of the swaps and discounts the cash flows back to present value using an appropriate discount rate. The discount rate is calculated by third party brokers according to the terms of the swap derivatives and evaluated by the Company for reasonableness, with consideration given to the potential non-performance risk by the Company. Future cash flows of the interest rate swap derivatives are equal to the fixed interest rate in the swap compared to the floating market interest rate multiplied by the notional amount for each period. To establish fair value for foreign currency derivatives, the Company uses forward market curves for Canadian dollars against the US dollar and multiplies the difference between the locked-in price and the market price by the notional amount of the derivative. Forward foreign currency market curves are provided by third party brokers. The Company's credit spread is factored into the locked-in price of the foreign exchange contracts. Deferred compensation assets and liabilities represent funds held by the Company in a Rabbi Trust for an executive deferral plan. These funds consist of actively traded equity and bond funds with quoted prices in active markets. The balance disclosed in the table above excludes cash and cash equivalents of $0.2 million as of September 30, 2017 and $0.4 million as of December 31, 2016 . Level 3 Fair Value Under the power exchange agreement the Company purchases power at a price that is based on the average operating and maintenance (O&M) charges from three surrogate nuclear power plants around the country. To estimate the fair value of this agreement, the Company estimates the difference between the purchase price based on the future O&M charges and forward prices for energy. The Company compares the Level 2 brokered quotes and forward price curves described above to an internally developed forward price which is based on the average O&M charges from the three surrogate nuclear power plants for the current year. Because the nuclear power plant O&M charges are only known for one year, all forward years are estimated assuming an annual escalation. In addition to the forward price being estimated using unobservable inputs, the Company also estimates the volumes of the transactions that will take place in the future based on historical average transaction volumes per delivery year (November to April). Significant increases or decreases in any of these inputs in isolation would result in a significantly higher or lower fair value measurement. Generally, a change in the current year O&M charges for the surrogate plants is accompanied by a directionally similar change in O&M charges in future years. There is generally not a correlation between external market prices and the O&M charges used to develop the internal forward price. For the power commodity option agreement, the Company uses the Black-Scholes-Merton valuation model to estimate the fair value, and this model includes significant inputs not observable or corroborated in the market. These inputs include: 1) the strike price (which is an internally derived price based on a combination of generation plant heat rate factors, natural gas market pricing, delivery and other O&M charges) and 2) estimated delivery volumes. Significant increases or decreases in these inputs in isolation would result in a significantly higher or lower fair value measurement. Generally, changes in overall commodity market prices are accompanied by directionally similar changes in the strike price assumptions used in the calculation. For the natural gas commodity exchange agreement, the Company uses the same Level 2 brokered quotes described above; however, the Company also estimates the purchase and sales volumes (within contractual limits) as well as the timing of those transactions. Changing the timing of volume estimates changes the timing of purchases and sales, impacting which brokered quote is used. Because the brokered quotes can vary significantly from period to period, the unobservable estimates of the timing and volume of transactions can have a significant impact on the calculated fair value. The Company currently estimates volumes and timing of transactions based on a most likely scenario using historical data. Historically, the timing and volume of transactions have not been highly correlated with market prices and market volatility. The following table presents the quantitative information which was used to estimate the fair values of the Level 3 assets and liabilities above as of September 30, 2017 (dollars in thousands): Fair Value (Net) at September 30, 2017 Valuation Technique Unobservable Input Range Power exchange agreement $ (16,654 ) Surrogate facility pricing O&M charges $38.87-$45.20/MWh (1) Escalation factor 5% - 2018 to 2019 Transaction volumes 396,984 - 432,357 MWhs Power option agreement $ (12 ) Black-Scholes- Merton Strike price $34.85/MWh - 2019 $47.54/MWh - 2018 Delivery volumes 157,517 - 222,683 MWhs Natural gas exchange agreement $ (3,599 ) Internally derived Forward purchase prices $1.70 - $2.24/mmBTU Forward sales prices $1.63 - $3.20/mmBTU Purchase volumes 115,000 - 310,000 mmBTUs Sales volumes 60,000 - 310,000 mmBTUs (1) The average O&M charges for the delivery year beginning in November 2017 are $41.95 per MWh. For ratemaking purposes the average O&M charges to be included for recovery in retail rates vary slightly between regulatory jurisdictions. The average O&M charges for the delivery year beginning in 2017 are $45.32 for Washington and $41.95 for Idaho. The valuation methods, significant inputs and resulting fair values described above were developed by the Company's management and are reviewed on at least a quarterly basis to ensure they provide a reasonable estimate of fair value each reporting period. The following table presents activity for energy commodity derivative assets (liabilities) measured at fair value using significant unobservable inputs (Level 3) for the three and nine months ended September 30 (dollars in thousands): Natural Gas Exchange Agreement Power Exchange Agreement Power Option Agreement Total Three months ended September 30, 2017: Balance as of July 1, 2017 $ (4,173 ) $ (13,784 ) $ (43 ) $ (18,000 ) Total gains or (losses) (realized/unrealized): Included in regulatory assets/liabilities (1) 617 (2,870 ) 31 (2,222 ) Settlements (43 ) — — (43 ) Ending balance as of September 30, 2017 (2) $ (3,599 ) $ (16,654 ) $ (12 ) $ (20,265 ) Three months ended September 30, 2016: Balance as of July 1, 2016 $ (6,857 ) $ (14,614 ) $ (105 ) $ (21,576 ) Total gains or (losses) (realized/unrealized): Included in regulatory assets/liabilities (1) 336 (1,696 ) (692 ) (2,052 ) Settlements — — — — Ending balance as of September 30, 2016 (2) $ (6,521 ) $ (16,310 ) $ (797 ) $ (23,628 ) Nine months ended September 30, 2017: Balance as of January 1, 2017 $ (5,885 ) $ (13,449 ) $ (76 ) $ (19,410 ) Total gains or (losses) (realized/unrealized): Included in regulatory assets/liabilities (1) 2,434 (8,035 ) 64 (5,537 ) Settlements (148 ) 4,830 — 4,682 Ending balance as of September 30, 2017 (2) $ (3,599 ) $ (16,654 ) $ (12 ) $ (20,265 ) Nine months ended September 30, 2016: Balance as of January 1, 2016 $ (5,039 ) $ (21,961 ) $ (124 ) $ (27,124 ) Total gains or (losses) (realized/unrealized): Included in regulatory assets/liabilities (1) (2,960 ) 272 (673 ) (3,361 ) Settlements 1,478 5,379 — 6,857 Ending balance as of September 30, 2016 (2) $ (6,521 ) $ (16,310 ) $ (797 ) $ (23,628 ) (1) All gains and losses are included in other regulatory assets and liabilities. There were no gains and losses included in either net income or other comprehensive income during any of the periods presented in the table above. (2) There were no purchases, issuances or transfers from other categories of any derivatives instruments during the periods presented in the table above.

Common Stock Common Stock (Note

Common Stock Common Stock (Notes)	9 Months Ended
	Sep. 30, 2017
Stockholders' Equity Note [Abstract]
Stockholders' Equity Note Disclosure [Text Block]	COMMON STOCK The Company has entered into four separate sales agency agreements under which Avista Corp.'s sales agents may offer and sell up to 3.8 million new shares of Avista Corp.'s common stock, no par value, from time to time. The sales agency agreements expire on February 29, 2020. As of September 30, 2017 , 1.6 million shares have been issued under these agreements, leaving 2.2 million shares remaining to be issued. No shares were issued under these agreements in the nine months ended September 30, 2017 . In the nine months ended September 30, 2017 , Avista Corp. issued 0.2 million shares of common stock, most of which were under employee incentive plans. The Company also issued a small number of shares under the 401(k) employee investment plan. Total net proceeds for all issuances were $1.5 million .

Earnings Per Common Share Attri

Earnings Per Common Share Attributable To Avista Corporation	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Earnings Per Common Share Attributable To Avista Corporation Shareholders	EARNINGS PER COMMON SHARE ATTRIBUTABLE TO AVISTA CORP. SHAREHOLDERS The following table presents the computation of basic and diluted earnings per common share attributable to Avista Corp. shareholders for the three and nine months ended September 30 (in thousands, except per share amounts): Three months ended September 30, Nine months ended September 30, 2017 2016 2017 2016 Numerator: Net income attributable to Avista Corp. shareholders $ 4,451 $ 12,234 $ 88,338 $ 97,137 Denominator: Weighted-average number of common shares outstanding-basic 64,412 63,857 64,392 63,282 Effect of dilutive securities: Performance and restricted stock awards 480 468 246 405 Weighted-average number of common shares outstanding-diluted 64,892 64,325 64,638 63,687 Earnings per common share attributable to Avista Corp. shareholders: Basic $ 0.07 $ 0.19 $ 1.37 $ 1.53 Diluted $ 0.07 $ 0.19 $ 1.37 $ 1.53 There were no shares excluded from the calculation because they were antidilutive.

Commitments And Contingencies

Commitments And Contingencies	9 Months Ended
	Sep. 30, 2017
Commitments and Contingencies Disclosure [Abstract]
Commitments And Contingencies	COMMITMENTS AND CONTINGENCIES In the course of its business, the Company becomes involved in various claims, controversies, disputes and other contingent matters, including the items described in this Note. Some of these claims, controversies, disputes and other contingent matters involve litigation or other contested proceedings. For all such matters, the Company intends to vigorously protect and defend its interests and pursue its rights. However, no assurance can be given as to the ultimate outcome of any particular matter because litigation and other contested proceedings are inherently subject to numerous uncertainties. For matters that affect Avista Utilities’ or AEL&P's operations, the Company intends to seek, to the extent appropriate, recovery of incurred costs through the ratemaking process. California Refund Proceeding In February 2016, APX, a market maker in the California Refund Proceedings in whose markets Avista Energy participated in the summer of 2000, asserted that Avista Energy and its other customer/participants may be responsible for a share of the disgorgement penalty APX may be found to owe to the California Parties (as defined in the 2016 Form 10-K). The penalty arises as a result of the Federal Energy and Regulatory Commission's (FERC) finding that APX committed violations in the California market in the summer of 2000. APX is making these assertions despite Avista Energy having been dismissed in FERC Opinion No. 536 from the on-going administrative proceeding at the FERC regarding potential wrongdoing in the California markets in the summer of 2000. APX has identified Avista Energy’s share of APX’s exposure to be as much as $16.0 million even though no wrongdoing allegations are specifically attributable to Avista Energy. Avista Energy believes its 2014 settlement with the California Parties insulates it from any such liability and that as a dismissed party it cannot be drawn back into the litigation. Avista Energy intends to vigorously dispute APX’s assertions of indirect liability, but cannot at this time predict the eventual outcome. Cabinet Gorge Total Dissolved Gas Abatement Plan Dissolved atmospheric gas levels (referred to as "Total Dissolved Gas" or "TDG") in the Clark Fork River exceed state of Idaho and federal water quality numeric standards downstream of Cabinet Gorge particularly during periods when excess river flows must be diverted over the spillway. Under the terms of the Clark Fork Settlement Agreement (CFSA) as incorporated in Avista Corp.’s FERC license for the Clark Fork Project, Avista Corp. has worked in consultation with agencies, tribes and other stakeholders to address this issue. Under the terms of a gas supersaturation mitigation plan, Avista is reducing TDG by constructing spill crest modifications on spill gates at the dam, and the Company expects to continue spill crest modifications over the next several years, in ongoing consultation with key stakeholders. Avista Corp. cannot at this time predict the outcome or estimate a range of costs associated with this contingency; however, the Company will continue to seek recovery, through the ratemaking process, of all operating and capitalized costs related to this issue. Fish Passage at Cabinet Gorge and Noxon Rapids In 1999, the United States Fish and Wildlife Service (USFWS) listed bull trout as threatened under the Endangered Species Act. In 2010, the USFWS issued a revised designation of critical habitat for bull trout, which includes the lower Clark Fork River. The USFWS issued a final recovery plan in October 2015. The CFSA describes programs intended to help restore bull trout populations in the project area. Using the concept of adaptive management and working closely with the USFWS, the Company evaluated the feasibility of fish passage at Cabinet Gorge and Noxon Rapids. The results of these studies led, in part, to the decision to move forward with development of permanent facilities, among other bull trout enhancement efforts. Parties to the CFSA have agreed to finalize the design of a fishway and plan its construction. Final design and construction costs are expected in late 2017 or early 2018. The Company believes its ongoing efforts through the CFSA continue to effectively address issues related to bull trout. Avista Corp. cannot at this time predict the outcome or estimate a range of costs associated with this contingency; however, the Company will continue to seek recovery, through the ratemaking process, of all operating and capitalized costs related to fish passage at Cabinet Gorge and Noxon Rapids. Legal Proceedings Related to the Pending Acquisition by Hydro One See Note 13 for information regarding the proposed acquisition of the Company by Hydro One. In connection with the proposed acquisition, as of the date of this quarterly report, three lawsuits have been filed in the United States District Court for the Eastern District of Washington, captioned as follows: • Jenβ v. Avista Corporation., et al. ,No. 2:17-cv-00333 (E.D. Wash.) (filed September 25, 2017); • Samuel v. Avista Corporation, et al ., No. 2:17-cv-00334 (E.D. Wash.) (filed September 26, 2017); and • Sharpenter v. Avista Corporation., et al. , No. 2:17-cv-00336 (E.D. Wash.) (filed September 26, 2017) In addition one lawsuit has been filed in the Superior Court for the State of Washington in and for Spokane County, captioned as follows: • Fink v. Morris, et al., No. 17203616-6 (filed September 15, 2017, amended complaint filed October 25, 2017). These lawsuits were filed against Avista Corporation, Hydro One Limited, Olympus Holding Corp. and Olympus Corp., as well as all members of the Company's Board of Directors, namely Erik Anderson, Kristianne Blake, Donald Burke, Rebecca Klein, Scott Maw, Scott Morris, Marc Racicot, Heidi Stanley, John Taylor and Janet Widmann, except that in Fink, Avista Corporation is not named and Bank of America Merrill Lynch is named as a defendant, and in the Jen β and Samuelson cases Hydro One Limited, Olympus Holding Corp., and Olympus Corp. are not named as defendants. The complaints generally allege, among other things, that the members of the Board breached their fiduciary duties by, among other things, conducting an allegedly inadequate sale process and agreeing to the acquisition at a price that allegedly undervalues Avista Corporation, and that Hydro One Limited, Olympus Holding Corp., and Olympus Corp. aided and abetted those purported breaches of duty. The aiding and abetting claims in Fink and Sharpenter were brought only against Hydro One Limited, Olympus Holding Corp. and Olympus Corp. The complaints also allege misstatements and omissions of material facts in the Company's proxy soliciting materials relating to the special meeting of shareholders to be held November 21, 2017 to approve the acquisition. The complaints seek various remedies, including an injunction against the acquisition and monetary damages, including attorneys’ fees and expenses. All defendants deny any wrongdoing in connection with the proposed acquisition and plan to vigorously defend against all pending claims; however, the Company cannot at this time predict the eventual outcome. Interest Rate Swaps included in the 2017 Washington General Rate Cases On October 27, 2017, UTC Staff and other parties to Avista Corp.'s electric and natural gas general rate cases filed their testimony. These parties recommended lower revenue requirements than what was proposed in Avista Corp.'s original filings. Additionally, the UTC Staff recommended the disallowance of the Washington portion of the Company's 2016 settled interest rate swaps. The total amount of the 2016 settled interest rate swaps was $54.0 million , with approximately 60 percent of this total being allocated to Washington. In addition to the settled interest rate swaps from 2016, the Company has a net regulatory asset of $8.8 million for interest rate swaps settled during the third quarter of 2017, and a net regulatory asset of $64.4 million for unsettled interest rate swaps as of September 30, 2017 related to forecasted debt issuances. Of those amounts, approximately 60 percent relate to Washington. If recovery of the 2016 settled interest rate swap payments referenced above is disallowed by the UTC, this could change the Company's current conclusion that settlement payments related to the 2017 settled interest rate swaps and the unsettled interest rate swaps are probable of recovery through rates. If the Company concluded that recovery of these swap related payments were no longer probable, the Company would be required to derecognize the related regulatory assets and liabilities with an adjustment through the income statement, and any subsequent gains and losses would be recognized through the income statement rather than recorded as a regulatory asset or liability. Interest rate swaps are a tool used throughout multiple industries to manage interest rate risk. They also provide certainty for future cash flows associated with future borrowings. Interest rate swap settlements have been included as a component of Avista Corp.’s cost of debt that has been approved by the UTC in past general rate cases. Accordingly, the Company still believes the interest rate swap payments are recoverable and will continue to work through the rate case process; however, the Company cannot predict the outcome of these rate cases and whether a disallowance will occur. Other Contingencies In the normal course of business, the Company has various other legal claims and contingent matters outstanding. The Company believes that any ultimate liability arising from these actions will not have a material impact on its financial condition, results of operations or cash flows. It is possible that a change could occur in the Company’s estimates of the probability or amount of a liability being incurred. Such a change, should it occur, could be significant. See "Note 19 of the Notes to Consolidated Financial Statements" in the 2016 Form 10-K for additional discussion regarding other contingencies.

Information By Business Segment

Information By Business Segments	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
Information by Business Segments	INFORMATION BY BUSINESS SEGMENTS The business segment presentation reflects the basis used by the Company's management to analyze performance and determine the allocation of resources. The Company's management evaluates performance based on income (loss) from operations before income taxes as well as net income (loss) attributable to Avista Corp. shareholders. The accounting policies of the segments are the same as those described in the summary of significant accounting policies. Avista Utilities' business is managed based on the total regulated utility operation; therefore, it is considered one segment. AEL&P is a separate reportable business segment as it has separate financial reports that are reviewed in detail by the Chief Operating Decision Maker and its operations and risks are sufficiently different from Avista Utilities and the other businesses at AERC that it cannot be aggregated with any other operating segments. The Other category, which is not a reportable segment, includes other investments and operations of various subsidiaries, as well as certain other operations of Avista Capital. The following table presents information for each of the Company’s business segments (dollars in thousands): Avista Utilities Alaska Electric Light and Power Company Total Utility Other Intersegment Eliminations (1) Total For the three months ended September 30, 2017: Operating revenues $ 280,776 $ 10,864 $ 291,640 $ 5,456 $ — $ 297,096 Resource costs 104,516 4,052 108,568 — — 108,568 Other operating expenses (2) 81,045 3,469 84,514 6,598 — 91,112 Depreciation and amortization 41,516 1,452 42,968 137 — 43,105 Income (loss) from operations 31,023 1,298 32,321 (1,279 ) — 31,042 Interest expense (3) 23,132 895 24,027 215 (71 ) 24,171 Income taxes 5,972 44 6,016 (863 ) — 5,153 Net income (loss) attributable to Avista Corp. shareholders 5,419 427 5,846 (1,395 ) — 4,451 Capital expenditures (4) 109,066 1,073 110,139 1,050 — 111,189 Avista Utilities Alaska Electric Light and Power Company Total Utility Other Intersegment Eliminations (1) Total For the three months ended September 30, 2016: Operating revenues $ 287,193 $ 9,796 $ 296,989 $ 6,360 $ — $ 303,349 Resource costs 115,228 3,509 118,737 — — 118,737 Other operating expenses 72,318 2,842 75,160 6,756 — 81,916 Depreciation and amortization 38,909 1,331 40,240 193 — 40,433 Income (loss) from operations 38,554 1,629 40,183 (589 ) — 39,594 Interest expense (3) 20,772 894 21,666 149 (19 ) 21,796 Income taxes 7,983 339 8,322 (716 ) — 7,606 Net income (loss) attributable to Avista Corp. shareholders 12,673 866 13,539 (1,305 ) — 12,234 Capital expenditures (4) 101,558 3,699 105,257 105 — 105,362 For the nine months ended September 30, 2017: Operating revenues $ 992,904 $ 38,002 $ 1,030,906 $ 17,161 $ — $ 1,048,067 Resource costs 366,590 10,315 376,905 — — 376,905 Other operating expenses (2) 231,727 9,236 240,963 19,863 — 260,826 Depreciation and amortization 123,249 4,347 127,596 482 — 128,078 Income (loss) from operations 193,629 12,080 205,709 (3,184 ) — 202,525 Interest expense (3) 68,641 2,684 71,325 558 (112 ) 71,771 Income taxes 49,881 3,582 53,463 (1,915 ) — 51,548 Net income (loss) attributable to Avista Corp. shareholders 85,623 5,961 91,584 (3,246 ) — 88,338 Capital expenditures (4) 283,081 4,772 287,853 1,219 — 289,072 For the nine months ended September 30, 2016: Operating revenues $ 989,981 $ 32,689 $ 1,022,670 $ 17,690 $ — $ 1,040,360 Resource costs 380,913 9,358 390,271 — — 390,271 Other operating expenses 221,364 8,241 229,605 18,862 — 248,467 Depreciation and amortization 115,126 3,984 119,110 573 — 119,683 Income (loss) from operations 199,661 9,354 209,015 (1,745 ) — 207,270 Interest expense (3) 61,652 2,684 64,336 459 (116 ) 64,679 Income taxes 53,004 2,910 55,914 (1,253 ) — 54,661 Net income (loss) attributable to Avista Corp. shareholders 94,431 4,885 99,316 (2,179 ) — 97,137 Capital expenditures (4) 274,041 14,031 288,072 270 — 288,342 Total Assets: As of September 30, 2017: $ 5,116,059 $ 276,426 $ 5,392,485 $ 59,747 $ — $ 5,452,232 As of December 31, 2016: $ 4,975,555 $ 273,770 $ 5,249,325 $ 60,430 $ — $ 5,309,755 (1) Intersegment eliminations reported as interest expense represent intercompany interest. (2) Other operating expenses for Avista Utilities for the three and nine months ended September 30, 2017 include acquisition costs of $6.7 million and $8.0 million , respectively, which are separately disclosed on the Condensed Consolidated Statements of Income. (3) Including interest expense to affiliated trusts. (4) The capital expenditures for the other businesses are included in other investing activities on the Condensed Consolidated Statements of Cash Flows.

Pending Merger with Hydro One P

Pending Merger with Hydro One Pending Merger with Hydro One (Notes)	9 Months Ended
	Sep. 30, 2017
Business Acquisition [Line Items]
Business Combination Disclosure [Text Block]	On July 19, 2017, Avista Corp. entered into a Merger Agreement, by and among Hydro One, Olympus Holding Corp., a wholly owned subsidiary of Hydro One (US parent), and Olympus Corp., a wholly owned subsidiary of US parent (Merger Sub). Subject to the terms and conditions of the Merger Agreement, Merger Sub will be merged with and into Avista Corp. (the "Merger"), with Avista Corp. surviving as an indirect, wholly-owned subsidiary of Hydro One. Hydro One, based in Toronto, is Ontario’s largest electricity transmission and distribution provider. At the effective time of the Merger, each share of Avista Corp. Common Stock issued and outstanding, other than Dissenting Shareholder Shares (as defined in the Merger Agreement) and shares of Avista Corp. Common Stock that are owned by Hydro One, US Parent or Merger Sub or any of their respective subsidiaries, will be converted automatically into the right to receive an amount in cash equal to $53.00 , without interest. Consummation of the Merger is subject to the satisfaction or waiver, if permissible under applicable law, of specified closing conditions, including, but not limited to, (i) the approval of the Merger by the holders of a majority of the outstanding shares of Avista Corp. Common Stock, (ii) the receipt of regulatory approvals required to consummate the Merger, including approval from the FERC, the Committee on Foreign Investment in the United States (CFIUS), the Federal Communications Commission (FCC), the UTC, IPUC, Public Service Commission of the State of Montana (MPSC), OPUC, and the RCA, and (iii) the expiration or termination of the applicable waiting period under the Hart-Scott-Rodino Antitrust Improvements Act of 1976, as amended. On September 14, 2017, Avista Corp. and Hydro One filed applications for approval of the acquisition with the FERC, the UTC, the IPUC, the OPUC, the MPSC and the RCA, requesting approval of the transaction on or before August 14, 2018. However, the OPUC has set a procedural schedule with an end date no later than September 14, 2018. As part of the applications for approval, Hydro One and Avista Corp. have proposed to flow through to Avista Corp.'s Washington, Idaho and Oregon retail customers, a rate credit totaling $31.5 million among the three jurisdictions over a 10-year period beginning at the time the Merger closes. In addition, to the extent Avista Corp. and Hydro One in a future rate proceeding demonstrate that cost savings, or benefits, directly related to the proposed transaction are already being flowed through to customers through base retail rates, the rate credit to customers would be reduced by an amount up to the $22.0 million over the 10-year period. The portion of the total rate credit that is not allowable for offset effectively represents acceptance by Hydro One of a lower rate of return during the 10-year period. On October 2, 2017, the Company distributed the definitive proxy statement associated with a special meeting of shareholders to be held on November 21, 2017 to vote on the transaction. The transaction is expected to close in the second half of 2018 subject to all of the above referenced approvals and the satisfaction or waiver of other specified conditions. The Merger Agreement also contains customary representations, warranties and covenants of Avista Corp., Hydro One, US Parent and Merger Sub. These covenants include, among others, an obligation on behalf of Avista Corp. to operate its business in the ordinary course until the Merger is consummated, subject to certain exceptions. In addition, the parties are required to use reasonable best efforts to obtain any required regulatory approvals. Avista Corp. has made certain additional customary covenants, including, among others, and subject to certain exceptions, a customary non-solicitation covenant prohibiting Avista Corp. from soliciting, providing non-public information or entering into discussions or negotiations concerning proposals relating to alternative business combination transactions, except as and to the extent permitted under the Merger Agreement with respect to an unsolicited written Takeover Proposal (as defined in the Merger Agreement) made prior to the approval of the Merger by Avista Corp.'s shareholders if, among other things, Avista Corp.'s board of directors determines in good faith that such Takeover Proposal is or could be reasonably expected to lead to a Superior Proposal (as defined in the Merger Agreement) and that failure to take such actions would reasonably be expected to be inconsistent with its fiduciary duties under applicable law. The Merger Agreement may be terminated by Avista Corp. and Hydro One by mutual consent and by either Avista Corp. or Hydro One under certain circumstances, including if the Merger is not consummated by September 30, 2018 (subject to an extension of up to six months by either party if all of the conditions to closing, other than the conditions related to obtaining required regulatory approvals, the absence of a law or injunction preventing the consummation of the Merger and the absence of a Burdensome Condition (as defined in the Merger Agreement) in any required regulatory approval, have been satisfied). The Merger Agreement also provides for certain additional termination rights for each of Avista Corp. and Hydro One. Upon termination of the Merger Agreement under certain specified circumstances, including (i) termination by Avista Corp. in order to enter into a definitive agreement with respect to a Superior Proposal, or (ii) termination by Hydro One following a withdrawal by Avista Corp.'s board or directors of its recommendation of the Merger Agreement, Avista Corp. will be required to pay Hydro One a termination fee of $103.0 million (Company Termination Fee). Avista Corp. will also be required to pay Hydro One the Company Termination Fee in the event Avista Corp. signs or consummates any specified alternative transaction within twelve months following the termination of the Merger Agreement under certain circumstances. In addition, if the Merger Agreement is terminated under certain circumstances due to the failure to obtain required regulatory approvals, the imposition of a Burdensome Condition with respect to a required regulatory approval, or the breach by Hydro One, US Parent or Merger Sub of their obligations in respect of obtaining regulatory approvals, Hydro One will be required to pay Avista Corp. a termination fee of $103.0 million . See Note 11 for discussion of shareholder lawsuits filed against the Company, the Company’s directors, Hydro One Limited, Olympus Holding Corp., and Olympus Corp. in relation to the Merger Agreement and the proposed Merger. See also the Company's Current Report on Form 8-K, filed with the SEC on July 19, 2017, and its definitive proxy statement relating to the special meeting of shareholders to approve the proposed transaction, filed with the SEC on October 2, 2017. The Merger Agreement was filed as Exhibit 2.1 to such Current Report.

Summary Of Significant Accoun22

Summary Of Significant Accounting Policies (Policy)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Income Tax, Policy [Policy Text Block]	Effective Income Tax Rate For the three months ended September 30, 2017 and 2016 , the Company's effective tax rate was 53.6 percent and 38.3 percent , respectively. For the nine months ended September 30, 2017 and 2016 , the Company's effective tax rate was 36.9 percent and 36.0 percent , respectively. The effective tax rate increased during 2017 because the majority of acquisition costs, which reduce income before income taxes, are not deductible for tax purposes and thus do not reduce income tax expense.
Nature Of Business	Nature of Business Avista Corp. is primarily an electric and natural gas utility with certain other business ventures. Avista Utilities is an operating division of Avista Corp., comprising the regulated utility operations in the Pacific Northwest. Avista Utilities provides electric distribution and transmission, and natural gas distribution services in parts of eastern Washington and northern Idaho. Avista Utilities also provides natural gas distribution service in parts of northeastern and southwestern Oregon. Avista Utilities has electric generating facilities in Washington, Idaho, Oregon and Montana. Avista Utilities also supplies electricity to a small number of customers in Montana, most of whom are employees who operate Avista Utilities' Noxon Rapids generating facility. Alaska Energy and Resources Company (AERC) is a wholly-owned subsidiary of Avista Corp. The primary subsidiary of AERC is Alaska Electric Light and Power Company (AEL&P), which comprises Avista Corp.'s regulated utility operations in Alaska. Avista Capital, Inc. (Avista Capital), a wholly owned non-regulated subsidiary of Avista Corp., is the parent company of all of the subsidiary companies in the non-utility businesses, with the exception of AJT Mining Properties, Inc., which is a subsidiary of AERC. On July 19, 2017, Avista Corp. entered into an Agreement and Plan of Merger (Merger Agreement) to become a wholly-owned subsidiary of Hydro One Limited (Hydro One). Consummation of the pending acquisition is subject to a number of approvals and the satisfaction or waiver of other specified conditions. The transaction is expected to close in the second half of 2018. See Note 13 for additional information.
Basis Of Reporting	Basis of Reporting The condensed consolidated financial statements include the assets, liabilities, revenues and expenses of the Company and its subsidiaries and other majority owned subsidiaries and variable interest entities for which the Company or its subsidiaries are the primary beneficiaries. Intercompany balances were eliminated in consolidation. The accompanying condensed consolidated financial statements include the Company’s proportionate share of utility plant and related operations resulting from its interests in jointly owned plants.
Taxes Other Than Income Taxes	Taxes Other Than Income Taxes Taxes other than income taxes include state excise taxes, city occupational and franchise taxes, real and personal property taxes and certain other taxes not based on income. These taxes are generally based on revenues or the value of property. Utility-related taxes collected from customers (primarily state excise taxes and city utility taxes) are recorded as operating revenue and expense. Taxes other than income taxes consisted of the following items for the three and nine months ended September 30 (dollars in thousands): Three months ended September 30, Nine months ended September 30, 2017 2016 2017 2016 Utility-related taxes $ 12,663 $ 12,095 $ 47,799 $ 43,033 Property taxes 9,991 10,047 29,829 29,757 Other taxes 615 527 2,105 1,879 Total $ 23,269 $ 22,669 $ 79,733 $ 74,669
Materials And Supplies, Fuel Stock And Natural Gas Stored	Materials and Supplies, Fuel Stock and Stored Natural Gas Inventories of materials and supplies, fuel stock and stored natural gas are recorded at average cost for our regulated operations and the lower of cost or net realizable value for our non-regulated operations and consisted of the following as of September 30, 2017 and December 31, 2016 (dollars in thousands): September 30, December 31, 2017 2016 Materials and supplies $ 41,518 $ 40,700 Fuel stock 5,064 4,585 Stored natural gas 19,379 8,029 Total $ 65,961 $ 53,314
Derivative Assets And Liabilities	Derivative Assets and Liabilities Derivatives are recorded as either assets or liabilities on the Condensed Consolidated Balance Sheets measured at estimated fair value. The Washington Utilities and Transportation Commission (UTC) and the Idaho Public Utilities Commission (IPUC) issued accounting orders authorizing Avista Corp. to offset energy commodity derivative assets or liabilities with a regulatory asset or liability. This accounting treatment is intended to defer the recognition of mark-to-market gains and losses on energy commodity transactions until the period of delivery. Realized benefits and costs result in adjustments to retail rates through purchased gas cost adjustments, the Energy Recovery Mechanism (ERM) in Washington, the Power Cost Adjustment (PCA) mechanism in Idaho, and periodic general rate cases. The resulting regulatory assets have been concluded to be probable of recovery through future rates. Substantially all forward contracts to purchase or sell power and natural gas are recorded as derivative assets or liabilities at estimated fair value with an offsetting regulatory asset or liability. Contracts that are not considered derivatives are accounted for on the accrual basis until they are settled or realized unless there is a decline in the fair value of the contract that is determined to be other-than-temporary. For interest rate swap derivatives, Avista Corp. records all mark-to-market gains and losses in each accounting period as assets and liabilities, as well as offsetting regulatory assets and liabilities, such that there is no income statement impact. The interest rate swap derivatives are risk management tools similar to energy commodity derivatives. Upon settlement of interest rate swap derivatives, the regulatory asset or liability is amortized as a component of interest expense over the term of the associated debt. The Company records an offset of interest rate swap derivative assets and liabilities with regulatory assets and liabilities, based on the prior practice of the commissions to provide recovery through the ratemaking process. As of September 30, 2017 , the Company has multiple master netting agreements with a variety of entities that allow for cross-commodity netting of derivative agreements with the same counterparty (i.e. power derivatives can be netted with natural gas derivatives). In addition, some master netting agreements allow for the netting of commodity derivatives and interest rate swap derivatives for the same counterparty. The Company does not have any agreements which allow for cross-affiliate netting among multiple affiliated legal entities. The Company nets all derivative instruments when allowed by the agreement for presentation in the Condensed Consolidated Balance Sheets.
Fair Value Measurements	Fair Value Measurements Fair value represents the price that would be received when selling an asset or paid to transfer a liability (an exit price) in an orderly transaction between market participants at the measurement date. Energy commodity derivative assets and liabilities, deferred compensation assets, as well as derivatives related to interest rate swaps and foreign currency exchange contracts, are reported at estimated fair value on the Condensed Consolidated Balance Sheets. See Note 8 for the Company’s fair value disclosures.
Contingencies	Contingencies The Company has unresolved regulatory, legal and tax issues which have inherently uncertain outcomes. The Company accrues a loss contingency if it is probable that a liability has been incurred and the amount of the loss or impairment can be reasonably estimated. The Company also discloses loss contingencies that do not meet these conditions for accrual if there is a reasonable possibility that a material loss may be incurred. As of September 30, 2017 , the Company has not recorded any significant amounts related to unresolved contingencies. See Note 11 for further discussion of the Company's commitments and contingencies.
Stockholders' Equity, Policy	Accumulated Other Comprehensive Loss Accumulated other comprehensive loss, net of tax, consisted of the following as of September 30, 2017 and December 31, 2016 (dollars in thousands): September 30, December 31, 2017 2016 Unfunded benefit obligation for pensions and other postretirement benefit plans - net of taxes of $3,780 and $4,075, respectively $ 7,020 $ 7,568 The following table details the reclassifications out of accumulated other comprehensive loss by component for the three and nine months ended September 30 (dollars in thousands). Amounts Reclassified from Accumulated Other Comprehensive Loss Three months ended September 30, Nine months ended September 30, Details about Accumulated Other Comprehensive Loss Components 2017 2016 2017 2016 Affected Line Item in Statement of Income Amortization of defined benefit pension items Amortization of net prior service cost $ (300 ) $ (312 ) $ (898 ) $ (934 ) (a) Amortization of net loss 3,637 3,642 $ 10,913 $ 10,926 (a) Adjustment due to effects of regulation (3,057 ) (3,115 ) (9,172 ) (11,453 ) (a) (b) 280 215 843 (1,461 ) Total before tax (98 ) (75 ) (295 ) 512 Tax benefit (expense) $ 182 $ 140 $ 548 $ (949 ) Net of tax (a) These accumulated other comprehensive loss components are included in the computation of net periodic pension cost (see Note 4 for additional details). (b) The adjustment for the effects of regulation during the nine months ended September 30, 2016 includes approximately $2.1 million related to the reclassification of a pension regulatory asset associated with one of our jurisdictions into accumulated other comprehensive loss.

Summary Of Significant Accoun23

Summary Of Significant Accounting Policies (Tables)	9 Months Ended
	Sep. 30, 2017
Accounting Policies [Abstract]
Schedule of Accumulated Other Comprehensive Income (Loss)	Accumulated Other Comprehensive Loss Accumulated other comprehensive loss, net of tax, consisted of the following as of September 30, 2017 and December 31, 2016 (dollars in thousands): September 30, December 31, 2017 2016 Unfunded benefit obligation for pensions and other postretirement benefit plans - net of taxes of $3,780 and $4,075, respectively $ 7,020 $ 7,568
Reclassifications Out of Accumulated Other Comprehensive Loss by Component	The following table details the reclassifications out of accumulated other comprehensive loss by component for the three and nine months ended September 30 (dollars in thousands). Amounts Reclassified from Accumulated Other Comprehensive Loss Three months ended September 30, Nine months ended September 30, Details about Accumulated Other Comprehensive Loss Components 2017 2016 2017 2016 Affected Line Item in Statement of Income Amortization of defined benefit pension items Amortization of net prior service cost $ (300 ) $ (312 ) $ (898 ) $ (934 ) (a) Amortization of net loss 3,637 3,642 $ 10,913 $ 10,926 (a) Adjustment due to effects of regulation (3,057 ) (3,115 ) (9,172 ) (11,453 ) (a) (b) 280 215 843 (1,461 ) Total before tax (98 ) (75 ) (295 ) 512 Tax benefit (expense) $ 182 $ 140 $ 548 $ (949 ) Net of tax (a) These accumulated other comprehensive loss components are included in the computation of net periodic pension cost (see Note 4 for additional details).
Utility Taxes	Utility-related taxes collected from customers (primarily state excise taxes and city utility taxes) are recorded as operating revenue and expense. Taxes other than income taxes consisted of the following items for the three and nine months ended September 30 (dollars in thousands): Three months ended September 30, Nine months ended September 30, 2017 2016 2017 2016 Utility-related taxes $ 12,663 $ 12,095 $ 47,799 $ 43,033 Property taxes 9,991 10,047 29,829 29,757 Other taxes 615 527 2,105 1,879 Total $ 23,269 $ 22,669 $ 79,733 $ 74,669
Materials And Supplies Fuel Stock And Natural Gas Stored	Inventories of materials and supplies, fuel stock and stored natural gas are recorded at average cost for our regulated operations and the lower of cost or net realizable value for our non-regulated operations and consisted of the following as of September 30, 2017 and December 31, 2016 (dollars in thousands): September 30, December 31, 2017 2016 Materials and supplies $ 41,518 $ 40,700 Fuel stock 5,064 4,585 Stored natural gas 19,379 8,029 Total $ 65,961 $ 53,314

Derivatives And Risk Manageme24

Derivatives And Risk Management (Tables)	9 Months Ended
	Sep. 30, 2017
Derivative Instruments and Hedges, Assets [Abstract]
Energy Commodity Derivatives	The following table presents the underlying energy commodity derivative volumes as of September 30, 2017 that are expected to be delivered in each respective year (in thousands of MWhs and mmBTUs): Purchases Sales Electric Derivatives Gas Derivatives Electric Derivatives Gas Derivatives Year Physical (1) MWH Financial (1) MWH Physical (1) mmBTUs Financial (1) mmBTUs Physical (1) Financial (1) Physical (1) Financial (1) Remainder 2017 191 303 5,018 33,165 160 367 1,519 17,878 2018 418 763 3,040 90,750 223 1,275 2,177 63,625 2019 235 737 610 44,290 158 982 1,345 30,050 2020 — — 910 7,750 — 246 1,430 — 2021 — — — — — — 1,049 — Thereafter — — — — — — — — The following table presents the underlying energy commodity derivative volumes as of December 31, 2016 that are expected to be delivered in each respective year (in thousands of MWhs and mmBTUs): Purchases Sales Electric Derivatives Gas Derivatives Electric Derivatives Gas Derivatives Year Physical (1) MWH Financial (1) MWH Physical (1) mmBTUs Financial (1) mmBTUs Physical (1) Financial (1) Physical (1) Financial (1) 2017 510 907 15,475 110,380 316 1,552 4,165 73,110 2018 397 — — 52,755 286 1,244 1,360 15,113 2019 235 — 610 29,475 158 982 1,345 4,020 2020 — — 910 2,725 — — 1,430 — 2021 — — — — — — 1,060 — Thereafter — — — — — — — —
Foreign Currency Exchange Contracts	The following table summarizes the foreign currency exchange derivatives that Avista Corp. has outstanding as of September 30, 2017 and December 31, 2016 (dollars in thousands): September 30, December 31, 2017 2016 Number of contracts 19 21 Notional amount (in United States dollars) $ 3,198 $ 2,819 Notional amount (in Canadian dollars) 3,925 3,754
Interest Rate Swap Agreements	The following table summarizes the unsettled interest rate swap derivatives that Avista Corp. has outstanding as of September 30, 2017 and December 31, 2016 (dollars in thousands): Balance Sheet Date Number of Contracts Notional Amount Mandatory Cash Settlement Date September 30, 2017 14 $ 275,000 2018 6 70,000 2019 3 30,000 2020 1 15,000 2021 5 60,000 2022 December 31, 2016 6 $ 75,000 2017 14 275,000 2018 6 70,000 2019 2 20,000 2020 5 60,000 2022
Derivative Instruments Summary	The following table presents the fair values and locations of derivative instruments recorded on the Condensed Consolidated Balance Sheet as of September 30, 2017 (in thousands): Fair Value as of September 30, 2017 Derivative and Balance Sheet Location Gross Asset Gross Liability Collateral Netted Net Asset (Liability) on Balance Sheet Foreign currency exchange derivatives Other current liabilities $ — $ (51 ) $ — $ (51 ) Interest rate swap derivatives Other current assets 2,637 (249 ) — 2,388 Other property and investments-net and other non-current assets 5,346 (1,896 ) — 3,450 Current interest rate swap derivative liabilities — (63,074 ) 28,554 (34,520 ) Non-current interest rate swap derivative liabilities — (7,136 ) 5,806 (1,330 ) Energy commodity derivatives Other current assets 197 (70 ) — 127 Current energy commodity derivative liabilities 23,904 (46,036 ) 11,078 (11,054 ) Other non-current liabilities, regulatory liabilities and deferred credits 13,891 (30,262 ) 4,569 (11,802 ) Total derivative instruments recorded on the balance sheet $ 45,975 $ (148,774 ) $ 50,007 $ (52,792 ) The following table presents the fair values and locations of derivative instruments recorded on the Condensed Consolidated Balance Sheet as of December 31, 2016 (in thousands): Fair Value as of December 31, 2016 Derivative and Balance Sheet Location Gross Asset Gross Liability Collateral Net Asset Foreign currency exchange derivatives Other current liabilities $ 5 $ (28 ) $ — $ (23 ) Interest rate swap derivatives Other current assets 3,393 — — 3,393 Other property and investments-net and other non-current assets 5,754 (397 ) — 5,357 Current interest rate swap derivative liabilities — (15,756 ) 9,731 (6,025 ) Non-current interest rate swap derivative liabilities 3,951 (57,825 ) 25,169 (28,705 ) Energy commodity derivatives Other current assets 18,682 (16,787 ) — 1,895 Current energy commodity derivative liabilities 16,335 (29,598 ) 6,228 (7,035 ) Other non-current liabilities, regulatory liabilities and deferred credits 13,071 (29,990 ) 3,630 (13,289 ) Total derivative instruments recorded on the balance sheet $ 61,191 $ (150,381 ) $ 44,758 $ (44,432 )
Schedule of Assets Pledged as Collateral and Related Offsets [Table Text Block]	The following table presents Avista Corp.'s collateral outstanding related to its derivative instruments as of September 30, 2017 and December 31, 2016 (in thousands): September 30, December 31, 2017 2016 Energy commodity derivatives Cash collateral posted $ 19,570 $ 17,134 Letters of credit outstanding 31,500 24,400 Balance sheet offsetting (cash collateral against net derivative positions) 15,647 9,858 Interest rate swap derivatives Cash collateral posted 34,360 34,900 Letters of credit outstanding 6,000 3,600 Balance sheet offsetting (cash collateral against net derivative positions) 34,360 34,900 Certain of Avista Corp.’s derivative instruments contain provisions that require Avista Corp. to maintain an "investment grade" credit rating from the major credit rating agencies. If Avista Corp.’s credit ratings were to fall below "investment grade," it would be in violation of these provisions, and the counterparties to the derivative instruments could request immediate payment or demand immediate and ongoing collateralization on derivative instruments in net liability positions. The following table presents the aggregate fair value of all derivative instruments with credit-risk-related contingent features that are in a liability position and the amount of additional collateral Avista Corp. could be required to post as of September 30, 2017 and December 31, 2016 (in thousands): September 30, December 31, 2017 2016 Energy commodity derivatives Liabilities with credit-risk-related contingent features $ 670 $ 1,124 Additional collateral to post 670 1,046 Interest rate swap derivatives Liabilities with credit-risk-related contingent features 72,355 73,978 Additional collateral to post 12,730 21,100

Pension Plans And Other Postr25

Pension Plans And Other Postretirement Benefit Plans (Tables)	9 Months Ended
	Sep. 30, 2017
Components of Net Periodic Benefit Cost	Pension Benefits Other Post-retirement Benefits 2017 2016 2017 2016 Three months ended September 30: Service cost $ 5,092 $ 4,567 $ 799 $ 806 Interest cost 6,976 6,895 1,374 1,530 Expected return on plan assets (7,900 ) (6,887 ) (475 ) (465 ) Amortization of prior service cost — 1 (274 ) (300 ) Net loss recognition 2,517 2,161 1,168 1,453 Net periodic benefit cost $ 6,685 $ 6,737 $ 2,592 $ 3,024 Nine months ended September 30: Service cost $ 15,226 $ 13,655 $ 2,422 $ 2,389 Interest cost 20,903 20,695 4,147 4,623 Expected return on plan assets (23,700 ) (20,512 ) (1,425 ) (1,415 ) Amortization of prior service cost — 1 (898 ) (924 ) Net loss recognition 7,380 6,252 3,761 4,312 Net periodic benefit cost $ 19,809 $ 20,091 $ 8,007 $ 8,985

Committed Lines of Credit (Tabl

Committed Lines of Credit (Tables)	9 Months Ended
	Sep. 30, 2017
Short-term Debt [Abstract]
Schedule of Line of Credit Facilities [Table Text Block]	Borrowings outstanding and interest rates of borrowings (excluding letters of credit) under the Company’s revolving committed line of credit were as follows as of September 30, 2017 and December 31, 2016 (dollars in thousands): September 30, December 31, 2017 2016 Borrowings outstanding at end of period (1) $ 195,000 $ 120,000 Letters of credit outstanding at end of period $ 43,853 $ 34,353 Average interest rates at end of period 1.99 % 1.50 %

Long-Term Debt and Capital Le27

Long-Term Debt and Capital Leases Long-Term Debt and Capital Leases (Tables)	9 Months Ended
	Sep. 30, 2017
Debt Disclosure [Abstract]
Schedule of Long-term Debt Instruments [Table Text Block]	The following details long-term debt outstanding as of September 30, 2017 and December 31, 2016 (dollars in thousands): Maturity Year Description Interest Rate September 30, December 31, Avista Corp. Secured Long-Term Debt 2018 First Mortgage Bonds 5.95% $ 250,000 $ 250,000 2018 Secured Medium-Term Notes 7.39%-7.45% 22,500 22,500 2019 First Mortgage Bonds 5.45% 90,000 90,000 2020 First Mortgage Bonds 3.89% 52,000 52,000 2022 First Mortgage Bonds 5.13% 250,000 250,000 2023 Secured Medium-Term Notes 7.18%-7.54% 13,500 13,500 2028 Secured Medium-Term Notes 6.37% 25,000 25,000 2032 Secured Pollution Control Bonds (1) (1) 66,700 66,700 2034 Secured Pollution Control Bonds (1) (1) 17,000 17,000 2035 First Mortgage Bonds 6.25% 150,000 150,000 2037 First Mortgage Bonds 5.70% 150,000 150,000 2040 First Mortgage Bonds 5.55% 35,000 35,000 2041 First Mortgage Bonds 4.45% 85,000 85,000 2044 First Mortgage Bonds 4.11% 60,000 60,000 2045 First Mortgage Bonds 4.37% 100,000 100,000 2047 First Mortgage Bonds 4.23% 80,000 80,000 2051 First Mortgage Bonds 3.54% 175,000 175,000 Total Avista Corp. secured long-term debt 1,621,700 1,621,700 Alaska Electric Light and Power Company Secured Long-Term Debt 2044 First Mortgage Bonds 4.54% 75,000 75,000 Total secured long-term debt 1,696,700 1,696,700 Alaska Energy and Resources Company Unsecured Long-Term Debt 2019 Unsecured Term Loan 3.85% 15,000 15,000 Total secured and unsecured long-term debt 1,711,700 1,711,700 Other Long-Term Debt Components Capital lease obligations 62,969 65,435 Unamortized debt discount (668 ) (792 ) Unamortized long-term debt issuance costs (9,986 ) (10,639 ) Committed line of credit to be refinanced on a long-term basis (2) 89,100 — Total 1,853,115 1,765,704 Secured Pollution Control Bonds held by Avista Corporation (1) (83,700 ) (83,700 ) Current portion of long-term debt and capital leases (277,626 ) (3,287 ) Total long-term debt and capital leases $ 1,491,789 $ 1,678,717 (1) In December 2010, $66.7 million and $17.0 million of the City of Forsyth, Montana Pollution Control Revenue Refunding Bonds (Avista Corporation Colstrip Project) due in 2032 and 2034 , respectively, which had been held by Avista Corp. since 2008 and 2009, respectively, were refunded by new variable rate bond issues (Series 2010A and Series 2010B). The new bonds were not offered to the public and were purchased by Avista Corp. due to market conditions. The Company expects that at a later date, subject to market conditions, these bonds may be remarketed to unaffiliated investors. So long as Avista Corp. is the holder of these bonds, the bonds will not be reflected as an asset or a liability on Avista Corp.'s Consolidated Balance Sheets. (2) In September 2017, the Company entered into a bond purchase agreement with four institutional investors in the private placement market for the issuance and sale of $90.0 million of Avista Corp. first mortgage bonds in December 2017. The first mortgage bonds will bear a coupon rate of 3.91 percent and mature in December 2047 . The Company intends to use the $90.0 million of bond proceeds, less issuance costs, to refinance on a long-term basis $89.1 million outstanding under the Company's committed line of credit. As such, $89.1 million has been excluded from current liabilities and is recorded as long-term debt on the Condensed Consolidated Balance Sheets as of September 30, 2017 . In connection with the pricing of the first mortgage bonds, the Company cash-settled five interest rate swap derivatives (notional aggregate amount of $60.0 million ) and paid a net amount of $8.8 million .

Long- Term Debt to Affiliated28

Long- Term Debt to Affiliated Trust Long-Term Debt to Affiliated Trust (Tables)	9 Months Ended
	Sep. 30, 2017
Long-Term Debt to Affiliated Trust [Abstract]
Schedule Of Distribution Rates Paid [Table Text Block]	The distribution rates paid were as follows during the nine months ended September 30, 2017 and the year ended December 31, 2016 : September 30, December 31, 2017 2016 Low distribution rate 1.81 % 1.29 % High distribution rate 2.19 % 1.81 % Distribution rate at the end of the period 2.19 % 1.81 %

Fair Value (Tables)

Fair Value (Tables)	9 Months Ended
	Sep. 30, 2017
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair Value Inputs, Liabilities, Quantitative Information [Table Text Block]	The following table presents the quantitative information which was used to estimate the fair values of the Level 3 assets and liabilities above as of September 30, 2017 (dollars in thousands): Fair Value (Net) at September 30, 2017 Valuation Technique Unobservable Input Range Power exchange agreement $ (16,654 ) Surrogate facility pricing O&M charges $38.87-$45.20/MWh (1) Escalation factor 5% - 2018 to 2019 Transaction volumes 396,984 - 432,357 MWhs Power option agreement $ (12 ) Black-Scholes- Merton Strike price $34.85/MWh - 2019 $47.54/MWh - 2018 Delivery volumes 157,517 - 222,683 MWhs Natural gas exchange agreement $ (3,599 ) Internally derived Forward purchase prices $1.70 - $2.24/mmBTU Forward sales prices $1.63 - $3.20/mmBTU Purchase volumes 115,000 - 310,000 mmBTUs Sales volumes 60,000 - 310,000 mmBTUs
Carrying Value and Estimated Fair Value of Financial Instruments	The following table sets forth the carrying value and estimated fair value of the Company’s financial instruments not reported at estimated fair value on the Condensed Consolidated Balance Sheets as of September 30, 2017 and December 31, 2016 (dollars in thousands): September 30, 2017 December 31, 2016 Carrying Value Estimated Fair Value Carrying Value Estimated Fair Value Long-term debt (Level 2) $ 951,000 $ 1,072,557 $ 951,000 $ 1,048,661 Long-term debt (Level 3) 677,000 704,130 677,000 675,251 Snettisham capital lease obligation (Level 3) 60,349 62,000 62,160 62,800 Long-term debt to affiliated trusts (Level 3) 51,547 41,238 51,547 38,660
Fair Value of Assets And Liabilities Measured on Recurring Basis	Level 1 Level 2 Level 3 Counterparty Total September 30, 2017 Assets: Energy commodity derivatives $ — $ 37,902 $ — $ (37,775 ) $ 127 Level 3 energy commodity derivatives: Natural gas exchange agreement — — 90 (90 ) — Interest rate swap derivatives — 7,983 — (2,145 ) 5,838 Deferred compensation assets: Fixed income securities (2) 1,692 — — — 1,692 Equity securities (2) 6,276 — — — 6,276 Total $ 7,968 $ 45,885 $ 90 $ (40,010 ) $ 13,933 Liabilities: Energy commodity derivatives $ — $ 56,013 $ — $ (53,422 ) $ 2,591 Level 3 energy commodity derivatives: Natural gas exchange agreement — — 3,689 (90 ) 3,599 Power exchange agreement — — 16,654 — 16,654 Power option agreement — — 12 — 12 Foreign currency exchange derivatives — 51 — — 51 Interest rate swap derivatives — 72,355 — (36,505 ) 35,850 Total $ — $ 128,419 $ 20,355 $ (90,017 ) $ 58,757 Level 1 Level 2 Level 3 Counterparty Total December 31, 2016 Assets: Energy commodity derivatives $ — $ 47,994 $ — $ (46,099 ) $ 1,895 Level 3 energy commodity derivatives: Natural gas exchange agreement — — 69 (69 ) — Power exchange agreement — — 25 (25 ) — Foreign currency exchange derivatives — 5 — (5 ) — Interest rate swap derivatives — 13,098 — (4,348 ) 8,750 Deferred compensation assets: Fixed income securities (2) 1,789 — — — 1,789 Equity securities (2) 5,481 — — — 5,481 Total $ 7,270 $ 61,097 $ 94 $ (50,546 ) $ 17,915 Liabilities: Energy commodity derivatives $ — $ 56,871 $ — $ (55,957 ) $ 914 Level 3 energy commodity derivatives: Natural gas exchange agreement — — 5,954 (69 ) 5,885 Power exchange agreement — — 13,474 (25 ) 13,449 Power option agreement — — 76 — 76 Foreign currency exchange derivatives — 28 — (5 ) 23 Interest rate swap derivatives — 73,978 — (39,248 ) 34,730 Total $ — $ 130,877 $ 19,504 $ (95,304 ) $ 55,077 (1) The Company is permitted to net derivative assets and derivative liabilities with the same counterparty when a legally enforceable master netting agreement exists. In addition, the Company nets derivative assets and derivative liabilities against any payables and receivables for cash collateral held or placed with these same counterparties. (2) These assets are trading securities and are included in other property and investments-net and other non-current assets on the Condensed Consolidated Balance Sheets.
Reconciliation for All Assets Measured At Fair Value on a Recurring Basis Using Significant Unobservable Inputs (Level 3)	Natural Gas Exchange Agreement Power Exchange Agreement Power Option Agreement Total Three months ended September 30, 2017: Balance as of July 1, 2017 $ (4,173 ) $ (13,784 ) $ (43 ) $ (18,000 ) Total gains or (losses) (realized/unrealized): Included in regulatory assets/liabilities (1) 617 (2,870 ) 31 (2,222 ) Settlements (43 ) — — (43 ) Ending balance as of September 30, 2017 (2) $ (3,599 ) $ (16,654 ) $ (12 ) $ (20,265 ) Three months ended September 30, 2016: Balance as of July 1, 2016 $ (6,857 ) $ (14,614 ) $ (105 ) $ (21,576 ) Total gains or (losses) (realized/unrealized): Included in regulatory assets/liabilities (1) 336 (1,696 ) (692 ) (2,052 ) Settlements — — — — Ending balance as of September 30, 2016 (2) $ (6,521 ) $ (16,310 ) $ (797 ) $ (23,628 ) Nine months ended September 30, 2017: Balance as of January 1, 2017 $ (5,885 ) $ (13,449 ) $ (76 ) $ (19,410 ) Total gains or (losses) (realized/unrealized): Included in regulatory assets/liabilities (1) 2,434 (8,035 ) 64 (5,537 ) Settlements (148 ) 4,830 — 4,682 Ending balance as of September 30, 2017 (2) $ (3,599 ) $ (16,654 ) $ (12 ) $ (20,265 ) Nine months ended September 30, 2016: Balance as of January 1, 2016 $ (5,039 ) $ (21,961 ) $ (124 ) $ (27,124 ) Total gains or (losses) (realized/unrealized): Included in regulatory assets/liabilities (1) (2,960 ) 272 (673 ) (3,361 ) Settlements 1,478 5,379 — 6,857 Ending balance as of September 30, 2016 (2) $ (6,521 ) $ (16,310 ) $ (797 ) $ (23,628 ) (1) All gains and losses are included in other regulatory assets and liabilities. There were no gains and losses included in either net income or other comprehensive income during any of the periods presented in the table above. (2) There were no purchases, issuances or transfers from other categories of any derivatives instruments during the periods presented in the table above.

Earnings Per Common Share Att30

Earnings Per Common Share Attributable To Avista Corporation (Tables)	9 Months Ended
	Sep. 30, 2017
Earnings Per Share [Abstract]
Computations Of Earnings Per Share	The following table presents the computation of basic and diluted earnings per common share attributable to Avista Corp. shareholders for the three and nine months ended September 30 (in thousands, except per share amounts): Three months ended September 30, Nine months ended September 30, 2017 2016 2017 2016 Numerator: Net income attributable to Avista Corp. shareholders $ 4,451 $ 12,234 $ 88,338 $ 97,137 Denominator: Weighted-average number of common shares outstanding-basic 64,412 63,857 64,392 63,282 Effect of dilutive securities: Performance and restricted stock awards 480 468 246 405 Weighted-average number of common shares outstanding-diluted 64,892 64,325 64,638 63,687 Earnings per common share attributable to Avista Corp. shareholders: Basic $ 0.07 $ 0.19 $ 1.37 $ 1.53 Diluted $ 0.07 $ 0.19 $ 1.37 $ 1.53 There were no shares excluded from the calculation because they were antidilutive.

Information By Business Segme31

Information By Business Segments (Tables)	9 Months Ended
	Sep. 30, 2017
Segment Reporting [Abstract]
Information by Business Segments	The following table presents information for each of the Company’s business segments (dollars in thousands): Avista Utilities Alaska Electric Light and Power Company Total Utility Other Intersegment Eliminations (1) Total For the three months ended September 30, 2017: Operating revenues $ 280,776 $ 10,864 $ 291,640 $ 5,456 $ — $ 297,096 Resource costs 104,516 4,052 108,568 — — 108,568 Other operating expenses (2) 81,045 3,469 84,514 6,598 — 91,112 Depreciation and amortization 41,516 1,452 42,968 137 — 43,105 Income (loss) from operations 31,023 1,298 32,321 (1,279 ) — 31,042 Interest expense (3) 23,132 895 24,027 215 (71 ) 24,171 Income taxes 5,972 44 6,016 (863 ) — 5,153 Net income (loss) attributable to Avista Corp. shareholders 5,419 427 5,846 (1,395 ) — 4,451 Capital expenditures (4) 109,066 1,073 110,139 1,050 — 111,189 Avista Utilities Alaska Electric Light and Power Company Total Utility Other Intersegment Eliminations (1) Total For the three months ended September 30, 2016: Operating revenues $ 287,193 $ 9,796 $ 296,989 $ 6,360 $ — $ 303,349 Resource costs 115,228 3,509 118,737 — — 118,737 Other operating expenses 72,318 2,842 75,160 6,756 — 81,916 Depreciation and amortization 38,909 1,331 40,240 193 — 40,433 Income (loss) from operations 38,554 1,629 40,183 (589 ) — 39,594 Interest expense (3) 20,772 894 21,666 149 (19 ) 21,796 Income taxes 7,983 339 8,322 (716 ) — 7,606 Net income (loss) attributable to Avista Corp. shareholders 12,673 866 13,539 (1,305 ) — 12,234 Capital expenditures (4) 101,558 3,699 105,257 105 — 105,362 For the nine months ended September 30, 2017: Operating revenues $ 992,904 $ 38,002 $ 1,030,906 $ 17,161 $ — $ 1,048,067 Resource costs 366,590 10,315 376,905 — — 376,905 Other operating expenses (2) 231,727 9,236 240,963 19,863 — 260,826 Depreciation and amortization 123,249 4,347 127,596 482 — 128,078 Income (loss) from operations 193,629 12,080 205,709 (3,184 ) — 202,525 Interest expense (3) 68,641 2,684 71,325 558 (112 ) 71,771 Income taxes 49,881 3,582 53,463 (1,915 ) — 51,548 Net income (loss) attributable to Avista Corp. shareholders 85,623 5,961 91,584 (3,246 ) — 88,338 Capital expenditures (4) 283,081 4,772 287,853 1,219 — 289,072 For the nine months ended September 30, 2016: Operating revenues $ 989,981 $ 32,689 $ 1,022,670 $ 17,690 $ — $ 1,040,360 Resource costs 380,913 9,358 390,271 — — 390,271 Other operating expenses 221,364 8,241 229,605 18,862 — 248,467 Depreciation and amortization 115,126 3,984 119,110 573 — 119,683 Income (loss) from operations 199,661 9,354 209,015 (1,745 ) — 207,270 Interest expense (3) 61,652 2,684 64,336 459 (116 ) 64,679 Income taxes 53,004 2,910 55,914 (1,253 ) — 54,661 Net income (loss) attributable to Avista Corp. shareholders 94,431 4,885 99,316 (2,179 ) — 97,137 Capital expenditures (4) 274,041 14,031 288,072 270 — 288,342 Total Assets: As of September 30, 2017: $ 5,116,059 $ 276,426 $ 5,392,485 $ 59,747 $ — $ 5,452,232 As of December 31, 2016: $ 4,975,555 $ 273,770 $ 5,249,325 $ 60,430 $ — $ 5,309,755 (1) Intersegment eliminations reported as interest expense represent intercompany interest. (2) Other operating expenses for Avista Utilities for the three and nine months ended September 30, 2017 include acquisition costs of $6.7 million and $8.0 million , respectively, which are separately disclosed on the Condensed Consolidated Statements of Income. (3) Including interest expense to affiliated trusts. (4) The capital expenditures for the other businesses are included in other investing activities on the Condensed Consolidated Statements of Cash Flows.

Summary Of Significant Accoun32

Summary Of Significant Accounting Policies (Utility Taxes) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accounting Policies [Abstract]
Utility-related taxes	$ 12,663	$ 12,095	$ 47,799	$ 43,033
Property taxes	9,991	10,047	29,829	29,757
Other taxes	615	527	2,105	1,879
Utilities Operating Expense, Taxes	$ 23,269	$ 22,669	$ 79,733	$ 74,669

Summary Of Significant Accoun33

Summary Of Significant Accounting Policies (Materials And Supplies Fuel Stock And Natural Gas Stored) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accounting Policies [Abstract]
Materials and supplies	$ 41,518	$ 40,700
Fuel stock	5,064	4,585
Natural gas stored	19,379	8,029
Total	$ 65,961	$ 53,314

Summary Of Significant Accoun34

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Accumulated Other Comprehensive Loss) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Accumulated Other Comprehensive Income (Loss), Pension and Other Postretirement Benefit Plans Net Unamortized (Gain) Loss, Tax	$ 3,780	$ 4,075
Accumulated Other Comprehensive (Income) Loss, Defined Benefit Plan, after Tax	7,020	7,568
Accumulated other comprehensive loss	$ (7,020)	$ (7,568)

Summary Of Significant Accoun35

Summary Of Significant Accounting Policies (Reclassifications Out of Accumulated Other Comprehensive Loss) (Details) - Reclassification Out of Accumulated Other Comprehensive Income [Member] - Accumulated Defined Benefit Plans Adjustment [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Amortization of net prior service cost	$ (300)	$ (312)	$ (898)	$ (934)
Amortization of net loss	3,637	3,642	10,913	10,926
Adjustment due to effects of regulation	(3,057)	(3,115)	(9,172)	(11,453)
Total before tax	280	215	843	(1,461)
Tax benefit (expense)	(98)	(75)	(295)	512
Net of tax	$ 182	$ 140	$ 548	(949)
OREGON
Reclassification Out of Accumulated Other Comprehensive Income [Line Items]
Adjustment due to effects of regulation				$ 2,100

Summary Of Significant Accoun36

Summary Of Significant Accounting Policies Summary of Significant Accounting Policies (Effective Income Tax Rate) (Details)	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Accounting Policies [Abstract]
Effective Income Tax Rate Reconciliation, Percent	53.60%	38.30%	36.90%	36.00%

New Accounting Standards New Ac

New Accounting Standards New Accounting Standards (Details) $ in Millions	9 Months Ended
	Sep. 30, 2016USD ($)
Accounting Changes and Error Corrections [Abstract]
Excess Tax Benefit Realized from Exercise of Stock Compensation Instruments	$ 1.6

Derivatives And Risk Manageme38

Derivatives And Risk Management (Narrative) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	$ 50,007	$ 44,758
Commodity Contracts [Member]
Derivative [Line Items]
Cash deposited as collateral	19,570	17,134
Letters of credit outstanding	31,500	24,400
Derivative, Fair Value, Amount Offset Against Collateral, Net	15,647	9,858
Liability position at aggregate fair value	670	1,124
Additional Collateral, Aggregate Fair Value	670	1,046
Interest Rate Swap [Member]
Derivative [Line Items]
Cash deposited as collateral	34,360	34,900
Letters of credit outstanding	6,000	3,600
Derivative, Fair Value, Amount Offset Against Collateral, Net	34,360	34,900
Liability position at aggregate fair value	72,355	73,978
Additional Collateral, Aggregate Fair Value	$ 12,730	$ 21,100

Derivatives And Risk Manageme39

Derivatives And Risk Management (Energy Commodity Derivatives) (Details) frequency in Thousands, Volt in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2017Voltfrequency	Dec. 31, 2016Voltfrequency
Sales [Member] | Physical [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2,017	160	316
2,018	223	286
2,019	158	158
2,020	0	0
2,021	0	0
Thereafter	0	0
Sales [Member] | Physical [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2017 | Volt	1,519	4,165
2018 | Volt	2,177	1,360
2019 | Volt	1,345	1,345
2,020	1,430	1,430
2,021	1,049	1,060
Thereafter | Volt	0	0
Sales [Member] | Financial [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2,017	367	1,552
2,018	1,275	1,244
2,019	982	982
2,020	246	0
2,021	0	0
Thereafter	0	0
Sales [Member] | Financial [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2017 | Volt	17,878	73,110
2018 | Volt	63,625	15,113
2019 | Volt	30,050	4,020
2020 | Volt	0	0
2,021	0	0
Thereafter | Volt	0	0
Purchase [Member] | Physical [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2,017	191	510
2,018	418	397
2,019	235	235
2,020	0	0
2,021	0	0
Thereafter	0	0
Purchase [Member] | Physical [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2017 | Volt	5,018	15,475
2018 | Volt	3,040	0
2019 | Volt	610	610
2020 | Volt	910	910
2021 | Volt	0	0
Thereafter | Volt	0	0
Purchase [Member] | Financial [Member] | Electric Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2,017	303	907
2,018	763	0
2,019	737	0
2,020	0	0
2,021	0	0
Thereafter	0	0
Purchase [Member] | Financial [Member] | Gas Derivative [Member]
Energy Commodity Derivative Volumes [Line Items]
2017 | Volt	33,165	110,380
2018 | Volt	90,750	52,755
2019 | Volt	44,290	29,475
2020 | Volt	7,750	2,725
2021 | Volt	0	0
Thereafter | Volt	0	0

Derivatives And Risk Manageme40

Derivatives And Risk Management Derivatives and Risk Management (Foreign Currency Exchange Contracts) (Details) CAD in Thousands, $ in Thousands	9 Months Ended
	Sep. 30, 2017CADderivative_contracts	Sep. 30, 2017USD ($)derivative_contracts	Dec. 31, 2016CADderivative_contracts	Dec. 31, 2016USD ($)derivative_contracts
Foreign Currency Fair Value Hedge Derivative [Line Items]
Number Of Days Canadian Currency Prices Are Settled With U.S. Dollars	60 days
Number of Foreign Currency Derivatives Held | derivative_contracts	19	19	21	21
United States of America, Dollars | Foreign Exchange Contract [Member]
Foreign Currency Fair Value Hedge Derivative [Line Items]
Derivative, Notional Amount | $		$ 3,198		$ 2,819
Canada, Dollars | Foreign Exchange Contract [Member]
Foreign Currency Fair Value Hedge Derivative [Line Items]
Derivative, Notional Amount | CAD	CAD 3,925		CAD 3,754

Derivatives And Risk Manageme41

Derivatives And Risk Management (Interest Rate Swap Agreements) (Details) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017USD ($)Caontracts	Sep. 30, 2017USD ($)Caontracts	Dec. 31, 2016USD ($)Caontracts
Derivatives, Fair Value [Line Items]
Payments for (proceeds from) derivative instrument operating activities	$ (8,800)
2017 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts			6
Derivative, Notional Amount			$ 75,000
Number of settled contracts | Caontracts	5
Settled Derivative, Notional Amount	$ 60,000
Derivative, Maturity Date			Dec. 31, 2017
2018 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	14	14	14
Derivative, Notional Amount	$ 275,000	$ 275,000	$ 275,000
Derivative, Maturity Date		Dec. 31, 2018	Dec. 31, 2018
2019 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	6	6	6
Derivative, Notional Amount	$ 70,000	$ 70,000	$ 70,000
Derivative, Maturity Date		Dec. 31, 2019	Dec. 31, 2019
2020 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	3	3	2
Derivative, Notional Amount	$ 30,000	$ 30,000	$ 20,000
Derivative, Maturity Date		Dec. 31, 2020	Dec. 31, 2020
2021 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	1	1
Derivative, Notional Amount	$ 15,000	$ 15,000
Derivative, Maturity Date		Dec. 31, 2021
2022 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	5	5	5
Derivative, Notional Amount	$ 60,000	$ 60,000	$ 60,000
Derivative, Maturity Date		Dec. 31, 2022	Dec. 31, 2022

Derivatives And Risk Manageme42

Derivatives And Risk Management (Derivative Instruments Summary) (Details) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Gross Asset	$ 45,975,000	$ 61,191,000
Gross Liability	(148,774,000)	(150,381,000)
Collateral Netted	50,007,000	44,758,000
Net Asset (Liability) on Balance Sheet	(52,792,000)	(44,432,000)
Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Collateral Netted	34,360,000	34,900,000
Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Collateral Netted	15,647,000	9,858,000
Other Current Liabilities [Member] | Foreign Exchange Contract [Member]
Derivatives, Fair Value [Line Items]
Gross Asset	0	5,000
Gross Liability	(51,000)	(28,000)
Collateral Netted	0	0
Net Asset (Liability) on Balance Sheet	(51,000)	(23,000)
Other Current Assets [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Gross Asset	2,637,000	3,393,000
Gross Liability	(249,000)	0
Collateral Netted	0	0
Net Asset (Liability) on Balance Sheet	2,388,000	3,393,000
Other Current Assets [Member] | Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Gross Asset	197,000	18,682,000
Gross Liability	(70,000)	(16,787,000)
Collateral Netted	0	0
Net Asset (Liability) on Balance Sheet	127,000	1,895,000
Other Current Liabilities [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Gross Asset	0	0
Gross Liability	(63,074,000)	(15,756,000)
Collateral Netted	28,554,000	9,731,000
Net Asset (Liability) on Balance Sheet	(34,520,000)	(6,025,000)
Non-Current Interest Rate Swap Derivative Liabilities [Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Gross Asset	0	3,951,000
Gross Liability	(7,136,000)	(57,825,000)
Collateral Netted	5,806,000	25,169,000
Net Asset (Liability) on Balance Sheet	(1,330,000)	(28,705,000)
Other Property And Investments Net and Other Non-current Assets[Member] | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Gross Asset	5,346,000	5,754,000
Gross Liability	(1,896,000)	(397,000)
Collateral Netted	0	0
Net Asset (Liability) on Balance Sheet	3,450,000	5,357,000
Current Utility Energy Commodity Derivative Liabilities [Member] | Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Gross Asset	23,904,000	16,335,000
Gross Liability	(46,036,000)	(29,598,000)
Collateral Netted	11,078,000	6,228,000
Net Asset (Liability) on Balance Sheet	(11,054,000)	(7,035,000)
Other Noncurrent Liabilities [Member] | Commodity Contracts [Member]
Derivatives, Fair Value [Line Items]
Gross Asset	13,891,000	13,071,000
Gross Liability	(30,262,000)	(29,990,000)
Collateral Netted	4,569,000	3,630,000
Net Asset (Liability) on Balance Sheet	$ (11,802,000)	$ (13,289,000)

Derivatives And Risk Manageme43

Derivatives And Risk Management Derivatives and Risk Management (Collateral) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Derivative [Line Items]
Derivative, Fair Value, Amount Offset Against Collateral, Net	$ 50,007	$ 44,758
Commodity Contracts [Member]
Derivative [Line Items]
Liability position at aggregate fair value	670	1,124
Additional Collateral, Aggregate Fair Value	670	1,046
Collateral Already Posted, Aggregate Fair Value	19,570	17,134
Letters of credit outstanding	31,500	24,400
Derivative, Fair Value, Amount Offset Against Collateral, Net	15,647	9,858
Foreign Exchange Contract [Member]
Derivative [Line Items]
Collateral Already Posted, Aggregate Fair Value	0	0
Letters of credit outstanding	$ 0	$ 0

Pension Plans And Other Postr44

Pension Plans And Other Postretirement Benefit Plans (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Payment for Pension Benefits	$ 22,000	$ 12,000	$ 12,000

Pension Plans And Other Postr45

Pension Plans And Other Postretirement Benefit Plans (Components Of Net Periodic Benefit Cost) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Percentage of Net Periodic Benefit Costs Capitalized to Utility Property			40.00%
Percentage of Net Periodic Benefit Costs Recorded to Operating Expenses			60.00%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 5,092	$ 4,567	$ 15,226	$ 13,655
Interest cost	6,976	6,895	20,903	20,695
Expected return on plan assets	(7,900)	(6,887)	(23,700)	(20,512)
Amortization of prior service cost	0	1	0	1
Net loss recognition	2,517	2,161	7,380	6,252
Net periodic benefit cost	6,685	6,737	19,809	20,091
Other Post-Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	799	806	2,422	2,389
Interest cost	1,374	1,530	4,147	4,623
Expected return on plan assets	(475)	(465)	(1,425)	(1,415)
Amortization of prior service cost	(274)	(300)	(898)	(924)
Net loss recognition	1,168	1,453	3,761	4,312
Net periodic benefit cost	$ 2,592	$ 3,024	$ 8,007	$ 8,985

Committed Lines of Credit (Deta

Committed Lines of Credit (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Jul. 01, 2014	Apr. 30, 2014
Avista Utilities [Member]
Short-term Debt [Line Items]
Long-term Line of Credit	$ 195,000
Line of Credit, Current	105,900	$ 120,000
Long-term Line of Credit, Noncurrent	89,100	0
Line of Credit Facility, Maximum Borrowing Capacity				$ 400,000
Letters of credit outstanding at end of period	$ 43,853	$ 34,353
Average interest rate at end of period	1.99%	1.50%
Alaska Electric Light & Power [Member]
Short-term Debt [Line Items]
Line of Credit, Current	$ 0	$ 0
Line of Credit Facility, Maximum Borrowing Capacity			$ 25,000
Letters of credit outstanding at end of period	$ 0	$ 0

Long-Term Debt and Capital Le47

Long-Term Debt and Capital Leases Long-Term Debt and Capital Leases (Schedule of Long-Term Debt Instruments) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Payments for (proceeds from) derivative instrument operating activities	$ 8,800
Long-term Pollution Control Bond, Noncurrent	(83,700)	$ (83,700)	$ (83,700)
Secured Debt	1,696,700	1,696,700	1,696,700
Secured and Unsecured Debt	1,711,700	1,711,700	1,711,700
Capital Lease Obligations	62,969	62,969	65,435
Debt Instrument, Unamortized Discount	(668)	(668)	(792)
Unamortized Debt Issuance Expense	(9,986)	(9,986)	(10,639)
Long-Term Debt, Before Current Portion And Bonds Held by Company	1,853,115	1,853,115	1,765,704
Long-term Debt and Capital Lease Obligations, Current	(277,626)	(277,626)	(3,287)
Long-term debt and capital leases	1,491,789	1,491,789	1,678,717
Avista Utilities [Member]
Debt Instrument [Line Items]
Secured Debt	1,621,700	1,621,700	1,621,700
Long-term Line of Credit, Noncurrent	$ 89,100	$ 89,100	0
Avista Utilities [Member] | First Mortgage [Member] | 2051
Debt Instrument [Line Items]
Maturity Year		2,051
Interest Rate at End of Period	3.54%	3.54%
Secured Debt	$ 175,000	$ 175,000	175,000
Avista Utilities [Member] | First Mortgage [Member] | 2018 5.95%
Debt Instrument [Line Items]
Maturity Year		2,018
Interest Rate at End of Period	5.95%	5.95%
Secured Debt	$ 250,000	$ 250,000	250,000
Avista Utilities [Member] | First Mortgage [Member] | 2019
Debt Instrument [Line Items]
Maturity Year		2,019
Interest Rate at End of Period	5.45%	5.45%
Secured Debt	$ 90,000	$ 90,000	90,000
Avista Utilities [Member] | First Mortgage [Member] | 2020
Debt Instrument [Line Items]
Maturity Year		2,020
Interest Rate at End of Period	3.89%	3.89%
Secured Debt	$ 52,000	$ 52,000	52,000
Avista Utilities [Member] | First Mortgage [Member] | 2022
Debt Instrument [Line Items]
Maturity Year		2,022
Interest Rate at End of Period	5.13%	5.13%
Secured Debt	$ 250,000	$ 250,000	250,000
Avista Utilities [Member] | First Mortgage [Member] | 2035
Debt Instrument [Line Items]
Maturity Year		2,035
Interest Rate at End of Period	6.25%	6.25%
Secured Debt	$ 150,000	$ 150,000	150,000
Avista Utilities [Member] | First Mortgage [Member] | 2037
Debt Instrument [Line Items]
Maturity Year		2,037
Interest Rate at End of Period	5.70%	5.70%
Secured Debt	$ 150,000	$ 150,000	150,000
Avista Utilities [Member] | First Mortgage [Member] | 2040
Debt Instrument [Line Items]
Maturity Year		2,040
Interest Rate at End of Period	5.55%	5.55%
Secured Debt	$ 35,000	$ 35,000	35,000
Avista Utilities [Member] | First Mortgage [Member] | 2041
Debt Instrument [Line Items]
Maturity Year		2,041
Interest Rate at End of Period	4.45%	4.45%
Secured Debt	$ 85,000	$ 85,000	85,000
Avista Utilities [Member] | First Mortgage [Member] | 2044
Debt Instrument [Line Items]
Maturity Year		2,044
Interest Rate at End of Period	4.11%	4.11%
Secured Debt	$ 60,000	$ 60,000	60,000
Avista Utilities [Member] | First Mortgage [Member] | 2045
Debt Instrument [Line Items]
Maturity Year		2,045
Interest Rate at End of Period	4.37%	4.37%
Secured Debt	$ 100,000	$ 100,000	100,000
Avista Utilities [Member] | First Mortgage [Member] | 2047
Debt Instrument [Line Items]
Maturity Year		2,047
Interest Rate at End of Period	4.23%	4.23%
Secured Debt	$ 80,000	$ 80,000	80,000
Avista Utilities [Member] | First Mortgage [Member] | 2047 3.91 %
Debt Instrument [Line Items]
Maturity Year		12/1/2047
Interest Rate at End of Period	3.91%	3.91%
Secured Debt	$ 90,000	$ 90,000
Avista Utilities [Member] | Secured Debt [Member] | 2018 7.39% to 7.45%
Debt Instrument [Line Items]
Maturity Year		2,018
Medium-term Notes, Noncurrent	$ 22,500	$ 22,500	22,500
Avista Utilities [Member] | Secured Debt [Member] | 2018 7.39% to 7.45% | Minimum [Member]
Debt Instrument [Line Items]
Interest Rate at End of Period	7.39%	7.39%
Avista Utilities [Member] | Secured Debt [Member] | 2018 7.39% to 7.45% | Maximum [Member]
Debt Instrument [Line Items]
Interest Rate at End of Period	7.45%	7.45%
Avista Utilities [Member] | Secured Debt [Member] | 2023 7.18% to 7.54%
Debt Instrument [Line Items]
Maturity Year		2,023
Medium-term Notes, Noncurrent	$ 13,500	$ 13,500	13,500
Avista Utilities [Member] | Secured Debt [Member] | 2028
Debt Instrument [Line Items]
Maturity Year		2,028
Medium-term Notes, Noncurrent	$ 25,000	$ 25,000	25,000
Interest Rate at End of Period	6.37%	6.37%
Avista Utilities [Member] | Secured Debt [Member] | 2032
Debt Instrument [Line Items]
Maturity Year		2,032
Long-term Pollution Control Bond, Noncurrent	$ 66,700	$ 66,700	66,700
Avista Utilities [Member] | Secured Debt [Member] | 2034
Debt Instrument [Line Items]
Maturity Year		2,034
Long-term Pollution Control Bond, Noncurrent	$ 17,000	$ 17,000	17,000
Avista Utilities [Member] | Secured Debt [Member] | 2023 7.18% to 7.54% | Minimum [Member]
Debt Instrument [Line Items]
Interest Rate at End of Period	7.18%	7.18%
Avista Utilities [Member] | Secured Debt [Member] | 2023 7.18% to 7.54% | Maximum [Member]
Debt Instrument [Line Items]
Interest Rate at End of Period	7.54%	7.54%
Alaska Electric Light & Power [Member] | First Mortgage [Member] | 2044
Debt Instrument [Line Items]
Maturity Year		2,044
Interest Rate at End of Period	4.54%	4.54%
Secured Debt	$ 75,000	$ 75,000	75,000
Alaska Energy Resources Company [Member] | Unsecured Debt [Member] | 2019
Debt Instrument [Line Items]
Maturity Year		2,019
Interest Rate at End of Period	3.85%	3.85%
Unsecured Debt	$ 15,000	$ 15,000	$ 15,000

Long- Term Debt to Affiliated48

Long- Term Debt to Affiliated Trust Long-Term Debt to Affiliated Trust (Schedule of Distribution Rates Paid) (Details) - Trust Preferred Securities Subject to Mandatory Redemption [Member]	Sep. 30, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Interest Rate at End of Period	2.19%	1.81%
Minimum [Member]
Debt Instrument [Line Items]
Interest Rate at End of Period	1.81%	1.29%
Maximum [Member]
Debt Instrument [Line Items]
Interest Rate at End of Period	2.19%	1.81%

Long- Term Debt to Affiliated49

Long- Term Debt to Affiliated Trust Long-Term Debt to Affiliated Trust (Narrative) (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2000	Dec. 31, 1997
Debt Instrument [Line Items]
Payments for Repurchase of Trust Preferred Securities		$ 10
Equity Method Investment, Ownership Percentage	100.00%
Junior Subordinated Debenture Owed to Unconsolidated Subsidiary Trust	$ 51.5		$ 51.5
Trust Preferred Securities Subject to Mandatory Redemption [Member]
Debt Instrument [Line Items]
Proceeds from Issuance of Trust Preferred Securities			50
Debt Instrument, Description of Variable Rate Basis	LIBOR
Debt Instrument, Basis Spread on Variable Rate	0.875%
Common Trust Securities [Member]
Debt Instrument [Line Items]
Proceeds from Issuance of Trust Preferred Securities			$ 1.5

Fair Value (Carrying Value And

Fair Value (Carrying Value And Estimated Fair Value Of Financial Instruments) (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Capital Lease Obligations	$ 62,969	$ 65,435
Fair Value, Inputs, Level 2 [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	1,072,557	1,048,661
Fair Value, Inputs, Level 2 [Member] | Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	951,000	951,000
Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	704,130	675,251
Fair Value, Inputs, Level 3 [Member] | Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	$ 677,000	677,000
Secured and Unsecured Debt [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Inputs, Offered Quotes	$ 100
Alaska Electric Light & Power [Member] | Capital Lease Obligations [Member] | Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Capital Lease Obligations	$ 62,000	62,800
Alaska Electric Light & Power [Member] | Capital Lease Obligations [Member] | Fair Value, Inputs, Level 3 [Member] | Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Capital Lease Obligations	60,349	62,160
Affiliated Entity [Member] | Fair Value, Inputs, Level 3 [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	41,238	38,660
Affiliated Entity [Member] | Fair Value, Inputs, Level 3 [Member] | Reported Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Long-term Debt, Fair Value	$ 51,547	$ 51,547
Minimum [Member] | Secured and Unsecured Debt [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Inputs, Offered Quotes	$ 80
Maximum [Member] | Secured and Unsecured Debt [Member] | Estimate of Fair Value Measurement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Fair Value Inputs, Offered Quotes	$ 129.64

Fair Value (Fair Value Of Asset

Fair Value (Fair Value Of Assets And Liabilities Measured On Recurring Basis) (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Asset	$ 45,975	$ 61,191
Liability	148,774	150,381
Cash and cash equivalents	14,716	8,507	$ 7,084	$ 10,484
Fixed Income Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	200	400
Fair Value, Measurements, Recurring [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, assets	(40,010)	(50,546)
Interest rate swaps	5,838	8,750
Total	13,933	17,915
Counterparty and collateral netting, liabilities	(90,017)	(95,304)
Interest rate swaps	35,850	34,730
Total	58,757	55,077
Fair Value, Measurements, Recurring [Member] | Natural Gas Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, assets	(90)	(69)
Derivative Asset	0	0
Counterparty and collateral netting, liabilities	(90)	(69)
Derivative Liability	3,599	5,885
Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, assets		(25)
Derivative Asset		0
Counterparty and collateral netting, liabilities	0	(25)
Derivative Liability	16,654	13,449
Fair Value, Measurements, Recurring [Member] | Interest Rate Swap [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, assets	(2,145)	(4,348)
Counterparty and collateral netting, liabilities	(36,505)	(39,248)
Fair Value, Measurements, Recurring [Member] | Energy commodity derivatives [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, assets	(37,775)	(46,099)
Derivative Asset	127	1,895
Counterparty and collateral netting, liabilities	(53,422)	(55,957)
Derivative Liability	2,591	914
Fair Value, Measurements, Recurring [Member] | Power Option Agreement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, liabilities	0	0
Derivative Liability	12	76
Fair Value, Measurements, Recurring [Member] | Foreign Exchange Contract [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Counterparty and collateral netting, assets		(5)
Derivative Asset		0
Counterparty and collateral netting, liabilities	0	(5)
Derivative Liability	51	23
Fair Value, Measurements, Recurring [Member] | Fixed Income Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation assets:	1,692	1,789
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation assets:	6,276	5,481
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total	7,968	7,270
Total	0	0
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Fixed Income Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation assets:	1,692	1,789
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Equity Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deferred compensation assets:	6,276	5,481
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Interest rate swaps	7,983	13,098
Total	45,885	61,097
Interest rate swaps	72,355	73,978
Total	128,419	130,877
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Energy commodity derivatives [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Asset	37,902	47,994
Liability	56,013	56,871
Fair Value, Measurements, Recurring [Member] | Fair Value, Inputs, Level 2 [Member] | Foreign Exchange Contract [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Asset		5
Liability	51	28
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Total	90	94
Total	20,355	19,504
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Natural Gas Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Asset	90	69
Liability	3,689	5,954
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Power Exchange Agreements [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Gross Asset		25
Liability	16,654	13,474
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Power Option Agreement [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Liability	$ 12	$ 76

Fair Value (Quantitative Inform

Fair Value (Quantitative Information) (Details) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2017USD ($)$ / MWHMMBTUMWh$ / shares$ / MmBtu	Dec. 31, 2016USD ($)
Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Average operation and maintenance charges	41.95
Minimum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions Operation And Maintenance Charges	38.87
Transaction/Delivery Volumes | MWh	396,984
Minimum [Member] | Power Option Agreement [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Transaction/Delivery Volumes | MWh	157,517
Maximum [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions Operation And Maintenance Charges	45.20
Transaction/Delivery Volumes | MWh	432,357
Maximum [Member] | Power Option Agreement [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Transaction/Delivery Volumes | MWh	222,683
Fair Value, Measurements, Recurring [Member] | Power Exchange Agreements [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability | $	$ (16,654)	$ (13,449)
Fair Value, Measurements, Recurring [Member] | Power Option Agreement [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability | $	(12)	(76)
Fair Value, Measurements, Recurring [Member] | Natural Gas Exchange Agreements [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative Liability | $	$ (3,599)	$ (5,885)
WASHINGTON | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Average operation and maintenance charges	45.32
IDAHO | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Average operation and maintenance charges	41.95
Sales [Member] | Minimum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Forward Price | $ / MmBtu	1.63
Transaction/Delivery Volumes | MMBTU	60,000
Sales [Member] | Maximum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Forward Price | $ / MmBtu	3.20
Transaction/Delivery Volumes | MMBTU	310,000
Purchase [Member] | Minimum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Forward Price | $ / MmBtu	1.70
Transaction/Delivery Volumes | MMBTU	115,000
Purchase [Member] | Maximum [Member] | Natural Gas Exchange Agreements [Member] | Internally Derived Weighted Average Cost Of Gas [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Derivative, Forward Price | $ / MmBtu	2.24
Transaction/Delivery Volumes | MMBTU	310,000
2017 to 2019 [Member] | Power Exchange Agreements [Member] | Surrogate Facility Pricing [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Escalation Factor	5.00%
2019 | Power Option Agreement [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Exercise Price | $ / shares	$ 34.85
2018 | Power Option Agreement [Member] | Black Scholes Merton [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value Assumptions, Exercise Price | $ / shares	$ 47.54

Fair Value (Reconciliation For

Fair Value (Reconciliation For All Assets And Liabilities Measured At Fair Value On A Recurring Basis Using Significant Unobservable Inputs (Level 3)) (Details) - Fair Value, Inputs, Level 3 [Member] - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value Liabilities Measured On Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	$ (18,000)	$ (21,576)	$ (19,410)	$ (27,124)
Included in regulatory assets/liabilities	(2,222)	(2,052)	(5,537)	(3,361)
Settlements	(43)	0	4,682	6,857
Ending Balance	(20,265)	(23,628)	(20,265)	(23,628)
Natural Gas Exchange Agreements [Member]
Fair Value Liabilities Measured On Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(4,173)	(6,857)	(5,885)	(5,039)
Included in regulatory assets/liabilities	617	336	2,434	(2,960)
Settlements	(43)	0	(148)	1,478
Ending Balance	(3,599)	(6,521)	(3,599)	(6,521)
Power Option Agreement [Member]
Fair Value Liabilities Measured On Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(43)	(105)	(76)	(124)
Included in regulatory assets/liabilities	31	(692)	64	(673)
Settlements	0	0	0	0
Ending Balance	(12)	(797)	(12)	(797)
Power Exchange Agreements [Member]
Fair Value Liabilities Measured On Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Beginning Balance	(13,784)	(14,614)	(13,449)	(21,961)
Included in regulatory assets/liabilities	(2,870)	(1,696)	(8,035)	272
Settlements	0	0	4,830	5,379
Ending Balance	$ (16,654)	$ (16,310)	$ (16,654)	$ (16,310)

Common Stock Common Stock (Deta

Common Stock Common Stock (Details) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Shares Issued During Period, Value, Share-based Compensation	$ 1.5
Sales Agency Agreements Common Stock, Shares Authorized	200,000,000	200,000,000
Document Period End Date	Sep. 30, 2017
Shares Issued During Period, Share-based Compensation	200,000
Sales Agency Agreement [Member]
Class of Stock [Line Items]
Shares issued from sales agency agreements	0	1,600,000
Sales Agency Agreements Common Stock, Shares Authorized	3,800,000
Common Stock Shares Authorized Under Sales Agency Agreements Remaining Shares Authorized To Sell	2,200,000

Earnings Per Share Attributable

Earnings Per Share Attributable To Avista Corporation (Computation Of Earnings Per Share) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Numerator:
Net income from continuing operations attributable to Avista Corp. shareholders	$ 4,451	$ 12,234	$ 88,338	$ 97,137
Denominator:
Weighted-average number of common shares outstanding-basic	64,412	63,857	64,392	63,282
Performance and restricted stock awards	480	468	246	405
Weighted-average number of common shares outstanding-diluted	64,892	64,325	64,638	63,687
Earnings Per Share, Basic [Abstract]
Basic	$ 0.07	$ 0.19	$ 1.37	$ 1.53
Earnings Per Share, Diluted [Abstract]
Diluted	$ 0.07	$ 0.19	$ 1.37	$ 1.53

Commitments And Contingencies (

Commitments And Contingencies (Details) $ in Millions	Sep. 30, 2017USD ($)
Loss Contingencies [Line Items]
Percent of Interest Rate Swaps Allocated to Washington	60.00%
Maximum [Member] | Market Manipulation Lawsuit [Member]
Loss Contingencies [Line Items]
Loss Contingency, Estimated Potential Loss	$ 16
2016 | Deferred Derivative Gain (Loss) [Member]
Loss Contingencies [Line Items]
Interest Rate Swap Regulatory Assets	54
2017 | Deferred Derivative Gain (Loss) [Member]
Loss Contingencies [Line Items]
Interest Rate Swap Regulatory Assets	8.8
2018 and Beyond | Deferred Derivative Gain (Loss) [Member]
Loss Contingencies [Line Items]
Interest Rate Swap Regulatory Assets	$ 64.4

Information By Business Segme57

Information By Business Segments (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017USD ($)	Sep. 30, 2016USD ($)	Sep. 30, 2017USD ($)Reportable_Segments	Sep. 30, 2016USD ($)	Dec. 31, 2016USD ($)
Segment Reporting Information [Line Items]
Acquisition costs	$ 6,730	$ 0	$ 8,004	$ 0
Number of Reportable Segments | Reportable_Segments			2
Operating revenues	297,096	303,349	$ 1,048,067	1,040,360
Resource costs	108,568	118,737	376,905	390,271
Other operating expenses	91,112	81,916	260,826	248,467
Depreciation and amortization	43,105	40,433	128,078	119,683
Income from operations	31,042	39,594	202,525	207,270
Interest expense	24,171	21,796	71,771	64,679
Income taxes	5,153	7,606	51,548	54,661
Net income from continuing operations attributable to Avista Corp. shareholders	4,451	12,234	88,338	97,137
Payments to Acquire Other Property, Plant, and Equipment	111,189	105,362	289,072	288,342
Total assets	5,452,232		5,452,232		$ 5,309,755
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	(71)	(19)	(112)	(116)
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	0		0		0
Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating revenues	291,640	296,989	1,030,906	1,022,670
Resource costs	108,568	118,737	376,905	390,271
Other operating expenses	84,514	75,160	240,963	229,605
Depreciation and amortization	42,968	40,240	127,596	119,110
Income from operations	32,321	40,183	205,709	209,015
Interest expense	24,027	21,666	71,325	64,336
Income taxes	6,016	8,322	53,463	55,914
Net income from continuing operations attributable to Avista Corp. shareholders	5,846	13,539	91,584	99,316
Payments to Acquire Other Property, Plant, and Equipment	110,139	105,257	287,853	288,072
Total assets	5,392,485		5,392,485		5,249,325
Operating Segments [Member] | Avista Utilities [Member]
Segment Reporting Information [Line Items]
Operating revenues	280,776	287,193	992,904	989,981
Resource costs	104,516	115,228	366,590	380,913
Other operating expenses	81,045	72,318	231,727	221,364
Depreciation and amortization	41,516	38,909	123,249	115,126
Income from operations	31,023	38,554	193,629	199,661
Interest expense	23,132	20,772	68,641	61,652
Income taxes	5,972	7,983	49,881	53,004
Net income from continuing operations attributable to Avista Corp. shareholders	5,419	12,673	85,623	94,431
Payments to Acquire Other Property, Plant, and Equipment	109,066	101,558	283,081	274,041
Total assets	5,116,059		5,116,059		4,975,555
Operating Segments [Member] | Alaska Electric Light & Power [Member]
Segment Reporting Information [Line Items]
Operating revenues	10,864	9,796	38,002	32,689
Resource costs	4,052	3,509	10,315	9,358
Other operating expenses	3,469	2,842	9,236	8,241
Depreciation and amortization	1,452	1,331	4,347	3,984
Income from operations	1,298	1,629	12,080	9,354
Interest expense	895	894	2,684	2,684
Income taxes	44	339	3,582	2,910
Net income from continuing operations attributable to Avista Corp. shareholders	427	866	5,961	4,885
Payments to Acquire Other Property, Plant, and Equipment	1,073	3,699	4,772	14,031
Total assets	276,426		276,426		273,770
Operating Segments [Member] | Corporate and Other [Member]
Segment Reporting Information [Line Items]
Operating revenues	5,456	6,360	17,161	17,690
Resource costs	0	0	0	0
Other operating expenses	6,598	6,756	19,863	18,862
Depreciation and amortization	137	193	482	573
Income from operations	(1,279)	(589)	(3,184)	(1,745)
Interest expense	215	149	558	459
Income taxes	(863)	(716)	(1,915)	(1,253)
Net income from continuing operations attributable to Avista Corp. shareholders	(1,395)	(1,305)	(3,246)	(2,179)
Payments to Acquire Other Property, Plant, and Equipment	1,050	$ 105	1,219	$ 270
Total assets	$ 59,747		$ 59,747		$ 60,430

Pending Merger with Hydro One58

Pending Merger with Hydro One Pending Merger with Hydro One (Details) - Hydro One [Member] - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2017	Jul. 19, 2017
Business Acquisition [Line Items]
Business Acquisition, Share Price		$ 53
Rate Credit to Retail Customers Related to Merger	$ 31.5
Non-Offsettable Rate Credit to Retail Customers Related to Merger	$ 22
Termination Fee Payable By Company If Pending Merger Is Terminated Under Certain Circumstances		$ 103