Alcoa (AA) 10-K 26 Feb 18 2017 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Billions	12 Months Ended
	Dec. 31, 2017	Feb. 20, 2018	Jun. 30, 2017
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2017
Document Fiscal Year Focus	2,017
Document Fiscal Period Focus	FY
Trading Symbol	AA
Entity Registrant Name	ALCOA CORP
Entity Central Index Key	1,675,149
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Large Accelerated Filer
Entity Common Stock, Shares Outstanding		186,175,616
Entity Public Float			$ 6

Statement of Consolidated Opera

Statement of Consolidated Operations - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Statement [Abstract]
Sales to unrelated parties	$ 10,788	$ 8,360	$ 10,121
Sales to related parties	864	958	1,078
Total sales (E)	11,652	9,318	11,199
Cost of goods sold (exclusive of expenses below)	9,072	7,898	9,039
Selling, general administrative, and other expenses	284	359	353
Research and development expenses	32	33	69
Provision for depreciation, depletion, and amortization	750	718	780
Restructuring and other charges (D)	309	318	983
Interest expense (S)	104	243	270
Other (income) expenses, net (S)	(58)	(89)	42
Total costs and expenses	10,493	9,480	11,536
Income (Loss) before income taxes	1,159	(162)	(337)
Provision for income taxes (P)	600	184	402
Net income (loss)	559	(346)	(739)
Less: Net income attributable to noncontrolling interest	342	54	124
Net Income (Loss) Attributable to Alcoa Corporation	$ 217	$ (400)	$ (863)
Earnings per Share Attributable to Alcoa Corporation Common Shareholders (F):
Basic	$ 1.18	$ (2.19)	$ (4.73)
Diluted	$ 1.16	$ (2.19)	$ (4.73)

Statement of Consolidated Compr

Statement of Consolidated Comprehensive (Loss) Income - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net income (loss)	$ 559	$ (346)	$ (739)
Other comprehensive (loss) income, net of tax (G):
Change in unrecognized net actuarial loss and prior service cost/benefit related to pension and other postretirement benefits	(447)	(202)	80
Foreign currency translation adjustments	284	315	(1,611)
Net change in unrecognized gains/losses on cash flow hedges	(1,089)	(342)	826
Total Other comprehensive (loss) income, net of tax	(1,252)	(229)	(705)
Comprehensive (loss) income	(693)	(575)	(1,444)
Alcoa Corporation [Member]
Net income (loss)	217	(400)	(863)
Other comprehensive (loss) income, net of tax (G):
Change in unrecognized net actuarial loss and prior service cost/benefit related to pension and other postretirement benefits	(456)	(202)	72
Foreign currency translation adjustments	188	213	(1,183)
Net change in unrecognized gains/losses on cash flow hedges	(1,139)	(346)	827
Total Other comprehensive (loss) income, net of tax	(1,407)	(335)	(284)
Comprehensive (loss) income	(1,190)	(735)	(1,147)
Non-controlling Interest [Member]
Net income (loss)	342	54	124
Other comprehensive (loss) income, net of tax (G):
Change in unrecognized net actuarial loss and prior service cost/benefit related to pension and other postretirement benefits	9		8
Foreign currency translation adjustments	96	102	(428)
Net change in unrecognized gains/losses on cash flow hedges	50	4	(1)
Total Other comprehensive (loss) income, net of tax	155	106	(421)
Comprehensive (loss) income	$ 497	$ 160	$ (297)

Consolidated Balance Sheet

Consolidated Balance Sheet - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents (O)	$ 1,358	$ 853
Receivables from customers	811	668
Other receivables	232	166
Inventories (I)	1,453	1,160
Fair value of derivative contracts (O)	113	51
Prepaid expenses and other current assets	271	283
Total current assets	4,238	3,181
Properties, plants, and equipment, net (J)	9,138	9,325
Investments (H)	1,410	1,358
Deferred income taxes (P)	814	741
Fair value of derivative contracts (O)	128	468
Other noncurrent assets (S)	1,719	1,668
Total Assets	17,447	16,741
Current liabilities:
Accounts payable, trade	1,898	1,455
Accrued compensation and retirement costs	459	456
Taxes, including income taxes	282	147
Fair value of derivative contracts (O)	185	35
Other current liabilities (C)	412	707
Long-term debt due within one year (L & O)	16	21
Total current liabilities	3,252	2,821
Long-term debt, less amount due within one year (L & O)	1,388	1,424
Accrued pension benefits (N)	2,341	1,851
Accrued other postretirement benefits (N)	1,100	1,166
Asset retirement obligations (Q)	617	604
Environmental remediation (R)	258	264
Fair value of derivative contracts (O)	1,105	234
Noncurrent income taxes (P)	309	310
Other noncurrent liabilities and deferred credits (S)	279	370
Total liabilities	10,649	9,044
Contingencies and commitments (R)
Alcoa Corporation shareholders' equity:
Common stock (M)	2	2
Additional capital	9,590	9,531
Retained earnings (deficit)	113	(104)
Accumulated other comprehensive loss (G)	(5,182)	(3,775)
Total Alcoa Corporation shareholders' equity	4,523	5,654
Noncontrolling interest (A)	2,275	2,043
Total equity	6,798	7,697
Total Liabilities and Equity	$ 17,447	$ 16,741

Statement of Consolidated Cash

Statement of Consolidated Cash Flows - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Cash from Operations
Net income (loss)	$ 559	$ (346)	$ (739)
Adjustments to reconcile net income (loss) to cash from operations:
Depreciation, depletion, and amortization	752	718	780
Deferred income taxes (P)	176	(46)	86
Equity earnings, net of dividends (H)	9	48	158
Restructuring and other charges (D)	309	318	983
Net gain from investing activities-asset sales (S)	(116)	(164)	(32)
Net periodic pension benefit cost (N)	111	66	67
Stock-based compensation (M)	24	28	35
Other	32	(16)	41
Changes in assets and liabilities, excluding effects of acquisitions, divestitures, and foreign currency translation adjustments:
(Increase) Decrease in receivables	(118)	(234)	130
(Increase) Decrease in inventories	(238)	1	212
Decrease (Increase) in prepaid expenses and other current assets	43	(52)	58
Increase (Decrease) in accounts payable, trade	377	6	(156)
(Decrease) in accrued expenses	(563)	(320)	(311)
Increase (Decrease) in taxes, including income taxes	111	(148)	(32)
Pension contributions (N)	(106)	(66)	(69)
(Increase) in noncurrent assets (R)	(99)	(184)	(356)
(Decrease) Increase in noncurrent liabilities	(39)	80	20
Cash provided from (used for) operations	1,224	(311)	875
Financing Activities
Net transfers from (to) former parent company		802	(34)
Cash paid to former parent company related to separation (A)	(247)	(1,072)
Net change in short-term borrowings (original maturities of three months or less)	7	(4)
Additions to debt (original maturities greater than three months)	21
Payments on debt (original maturities greater than three months) (L)	(60)	(34)	(24)
Proceeds from the exercise of employee stock options (M)	43	10
Contributions from noncontrolling interest (A)	80	48	2
Distributions to noncontrolling interest	(342)	(233)	(106)
Other	(8)
Cash used for financing activities	(506)	(483)	(162)
Investing Activities
Capital expenditures	(405)	(404)	(391)
Proceeds from the sale of assets and businesses (C)	245	112	70
Additions to investments (H)	(66)	(3)	(63)
Sales of investments (H)		146
Net change in restricted cash (L)		1,226
Cash (used for) provided from investing activities	(226)	1,077	(384)
Effect of exchange rate changes on cash and cash equivalents	13	13	(38)
Net change in cash and cash equivalents	505	296	291
Cash and cash equivalents at beginning of year	853	557	266
Cash and cash equivalents at end of year	$ 1,358	$ 853	$ 557

Statement of Changes in Consoli

Statement of Changes in Consolidated Equity - USD ($) $ in Millions	Total	Common Stock [Member]	Additional Capital [Member]	Retained (Deficit) Earnings [Member]	Accumulated Other Comprehensive Loss [Member]	Non-controlling Interest [Member]	ParentCo [Member]Parent Company Net Investment [Member]	Alcoa Corporation [Member]Retained (Deficit) Earnings [Member]
Balance at Dec. 31, 2014	$ 13,073				$ (1,316)	$ 2,474	$ 11,915
Net (loss) income	(739)					124	(863)
Other comprehensive (loss) income (G)	(705)				(284)	(421)
Change in Parent Company net investment	(10)						(10)
Contributions (A)	2					2
Distributions	(106)					(106)
Other	(2)					(2)
Balance at Dec. 31, 2015	11,513				(1,600)	2,071	11,042
Net (loss) income	(346)					54	(296)	$ (104)
Other comprehensive (loss) income (G)	(229)				(335)	106
Establishment of additional defined benefit plans (N)	(2,528)				(2,704)		176
Change in Parent Company net investment	(603)						(603)
Cash provided at separation to Parent Company (A)	(1,072)						(1,072)
Separation-related adjustments (A)	1,138		$ 9,521		864		$ (9,247)
Issuance of common stock (M)		$ 2	(2)
Stock-based compensation (M)	2		2
Common stock issued: compensation plans (M)	10		10
Contributions (A)	48					48
Distributions	(233)					(233)
Other	(3)					(3)
Balance at Dec. 31, 2016	7,697	2	9,531	$ (104)	(3,775)	2,043
Net (loss) income	559			217		342
Other comprehensive (loss) income (G)	(1,252)				(1,407)	155
Stock-based compensation (M)	24		24
Common stock issued: compensation plans (M)	43		43
Contributions (A)	80					80
Distributions	(342)					(342)
Other	11		8			3
Balance at Dec. 31, 2017	$ 6,798	$ 2	$ 9,590	$ 113	$ (5,182)	$ 2,275

Basis of Presentation

Basis of Presentation	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Basis of Presentation	A. Basis of Presentation Alcoa Corporation (or the “Company”) is a vertically integrated aluminum company comprised of bauxite mining, alumina refining, aluminum production (smelting, casting, and rolling), and energy generation. The Company has more than 40 operating locations (through direct and indirect ownership) in 10 countries around the world, situated primarily in Australia, Brazil, Canada, Europe, and the United States. References in these Notes to “ParentCo” refer to Alcoa Inc., a Pennsylvania corporation, and its consolidated subsidiaries (through October 31, 2016, at which time was renamed Arconic Inc. (Arconic)). Separation Transaction. The Separation Transaction was subject to a number of conditions, including, but not limited to: final approval by ParentCo’s Board of Directors (see below); the continuing validity of the private letter ruling from the Internal Revenue Service regarding certain U.S. federal income tax matters relating to the transaction; receipt of an opinion of legal counsel regarding the qualification of the distribution, together with certain related transactions, as a transaction that is generally tax-free On September 29, 2016, ParentCo’s Board of Directors approved the completion of the Separation Transaction by means of a pro rata distribution by ParentCo of 80.1% of the outstanding common stock of Alcoa Corporation to ParentCo shareholders of record as of the close of business on October 20, 2016 (the “Record Date”). Arconic was to retain the remaining 19.9% of Alcoa Corporation common stock. At the time of the Separation Transaction, ParentCo shareholders were to receive one share of Alcoa Corporation common stock for every three shares of ParentCo common stock held as of the close of business on the Record Date. ParentCo shareholders were to receive cash in lieu of fractional shares. In connection with the Separation Transaction, as of October 31, 2016, Alcoa Corporation entered into certain agreements with Arconic to implement the legal and structural separation between the two companies, govern the relationship between Alcoa Corporation and Arconic after the completion of the Separation Transaction, and allocate between Alcoa Corporation and Arconic various assets, liabilities and obligations, including, among other things, employee benefits, environmental liabilities, intellectual property, and tax-related Know-How, On November 1, 2016 (the “Separation Date”), the Separation Transaction was completed and became effective at 12:01 a.m. Eastern Standard Time. To effect the Separation Transaction, ParentCo undertook a series of transactions to separate the net assets and certain legal entities of ParentCo, resulting in a cash payment of $1,072 to ParentCo by Alcoa Corporation (an additional $247 was paid to Arconic by Alcoa Corporation in 2017, including $243 associated with the sale of certain of the Company’s energy operations —see Note C) with the net proceeds of a previous debt offering (see Note L). In conjunction with the Separation Transaction, 146,159,428 shares of Alcoa Corporation common stock were distributed to ParentCo shareholders. Additionally, Arconic retained 36,311,767 shares of Alcoa Corporation common stock representing its 19.9% retained interest (Arconic sold all of these shares in 2017). “Regular-way” ParentCo incurred costs to evaluate, plan, and execute the Separation Transaction, and Alcoa Corporation was allocated a pro rata portion of those costs based on segment revenue (see Cost Allocations below). ParentCo recognized $152 from January 2016 through October 2016 and $24 in 2015 for costs related to the Separation Transaction, of which $68 and $12, respectively, was allocated to Alcoa Corporation. The allocated amounts were included in Selling, general administrative, and other expenses on the accompanying Statement of Consolidated Operations. Basis of Presentation. Principles of Consolidation. AWAC is an unincorporated global joint venture between Alcoa Corporation and Alumina Limited of Australia (Alumina Limited) and consists of several affiliated operating entities, which own, or have an interest in, or operate the bauxite mines and alumina refineries within Alcoa Corporation’s Bauxite and Alumina segments (except for the Poços de Caldas mine and refinery and a portion of the São Luís refinery, all in Brazil) and the Portland smelter in Australia. Alcoa Corporation owns 60% and Alumina Limited owns 40% of these individual entities, which are consolidated by the Company for financial reporting purposes and include Alcoa of Australia Limited (AofA), Alcoa World Alumina LLC (AWA), and Alcoa World Alumina Brasil Ltda. (AWAB). Alumina Limited’s interest in the equity of such entities is reflected as Noncontrolling interest on the accompanying Consolidated Balance Sheet. In 2017, 2016, and 2015, AWAC received $80, $48, and $2, respectively, in contributions from Alumina Limited. Management evaluates whether an Alcoa Corporation entity or interest is a variable interest entity and whether Alcoa Corporation is the primary beneficiary. Consolidation is required if both of these criteria are met. Alcoa Corporation does not have any variable interest entities requiring consolidation. Prior to the Separation Date, Alcoa Corporation did not operate as a separate, standalone entity. Alcoa Corporation’s operations were included in ParentCo’s financial results. Accordingly, for all periods prior to the Separation Date, the accompanying Consolidated Financial Statements were prepared from ParentCo’s historical accounting records and were presented on a standalone basis as if Alcoa Corporation’s operations had been conducted independently from ParentCo. Such Consolidated Financial Statements include the historical operations that were considered to comprise Alcoa Corporation’s businesses, as well as certain assets and liabilities that were historically held at ParentCo’s corporate level but were specifically identifiable or otherwise attributable to Alcoa Corporation. ParentCo’s net investment in these operations is reflected as Parent Company net investment on the accompanying Consolidated Financial Statements. All significant transactions and accounts within Alcoa Corporation have been eliminated. All significant intercompany transactions between ParentCo and Alcoa Corporation were included within Parent Company net investment on the accompanying Consolidated Financial Statements. Cost Allocations. The Consolidated Financial Statements of Alcoa Corporation include general corporate expenses of ParentCo that were not historically charged to Alcoa Corporation for certain support functions that were provided on a centralized basis, such as expenses related to finance, audit, legal, information technology, human resources, communications, compliance, facilities, employee benefits and compensation, and research and development activities. These general corporate expenses were included on the accompanying Statement of Consolidated Operations within Cost of goods sold, Selling, general administrative and other expenses, and Research and development expenses. These expenses were allocated to Alcoa Corporation on the basis of direct usage when identifiable, with the remainder allocated based on Alcoa Corporation’s segment revenue as a percentage of ParentCo’s total segment revenue for both Alcoa Corporation and Arconic. All external debt not directly attributable to Alcoa Corporation was excluded from Alcoa Corporation’s Consolidated Balance Sheet. Financing costs related to these debt obligations were allocated to Alcoa Corporation based on the ratio of capital invested in Alcoa Corporation to the total capital invested by ParentCo in both Alcoa Corporation and Arconic, and were included on the accompanying Statement of Consolidated Operations within Interest expense. The following table reflects the allocations described above: 2016 2015 Cost of goods sold (1) $ 40 $ 93 Selling, general administrative, and other expenses (2) 150 146 Research and development expenses 2 17 Provision for depreciation, depletion, and amortization 18 22 Restructuring and other charges (3) 1 32 Interest expense 198 245 Other (income) expenses, net $ (7 ) $ 12 (1) Allocation principally relates to expenses for ParentCo’s retained pension and other postretirement benefits associated with closed and sold operations. (2) Allocation includes costs incurred by ParentCo associated with the Separation Transaction (see Separation Transaction above). (3) Allocation primarily relates to layoff programs for ParentCo corporate employees. Management believes the assumptions regarding the allocation of ParentCo’s general corporate expenses and financing costs were reasonable. Nevertheless, the Consolidated Financial Statements of Alcoa Corporation may not include all of the actual expenses that would have been incurred and may not reflect Alcoa Corporation’s consolidated results of operations, financial position, and cash flows had it been a standalone company during the periods prior to the Separation Date. Actual costs that would have been incurred if Alcoa Corporation had been a standalone company would depend on multiple factors, including organizational structure, capital structure, and strategic decisions made in various areas, including information technology and infrastructure. Transactions between Alcoa Corporation and ParentCo, including sales to Arconic, were included as related party transactions on the Consolidated Financial Statements and are considered to be effectively settled for cash at the time the transaction was recorded. The total net effect of the settlement of these transactions is reflected on the accompanying Statement of Consolidated Cash Flows as a financing activity and on Alcoa Corporation’s Consolidated Balance Sheet as Parent Company net investment. Cash Management. Cash was managed centrally with certain net earnings reinvested locally and working capital requirements met from existing liquid funds. Accordingly, the cash and cash equivalents held by ParentCo at the corporate level were not attributed to Alcoa Corporation for any of the periods prior to the Separation Date. Only cash amounts specifically attributable to Alcoa Corporation were reflected in the Company’s Consolidated Balance Sheet. Transfers of cash, both to and from ParentCo’s centralized cash management system, were reflected as a component of Parent Company net investment on Alcoa Corporation’s Consolidated Balance Sheet and as a financing activity on the accompanying Consolidated Statement of Cash Flows. ParentCo had an arrangement with several financial institutions to sell certain customer receivables without recourse on a revolving basis. The sale of such receivables was completed through the use of a bankruptcy-remote special-purpose entity, which was a consolidated subsidiary of ParentCo. In connection with this arrangement, certain of Alcoa Corporation’s customer receivables were sold on a revolving basis to this bankruptcy-remote subsidiary of ParentCo; these sales were reflected as a component of Parent Company net investment on Alcoa Corporation’s Consolidated Balance Sheet. ParentCo participated in several accounts payable settlement arrangements with certain vendors and third-party intermediaries. These arrangements provided that, at the vendor’s request, the third-party intermediary advance the amount of the scheduled payment to the vendor, less an appropriate discount, before the scheduled payment date and ParentCo made payment to the third-party intermediary on the date stipulated in accordance with the commercial terms negotiated with its vendors. In connection with these arrangements, certain of Alcoa Corporation’s accounts payable were settled, at the vendor’s request, before the scheduled payment date; these settlements were reflected as a component of Parent Company net investment on Alcoa Corporation’s Consolidated Balance Sheet. Related Party Transactions. Transactions between Alcoa Corporation and Arconic have been presented as related party transactions on the accompanying Consolidated Financial Statements. Sales to Arconic from Alcoa Corporation were $864, $958, and $1,078 in 2017, 2016, and 2015, respectively. As of December 31, 2017 and 2016, outstanding receivables from Arconic were $84 and $67, respectively, and were included in Receivables from customers on the accompanying Consolidated Balance Sheet.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	B. Summary of Significant Accounting Policies Cash Equivalents. Inventory Valuation. last-in, first-out Properties, Plants, and Equipment. one-year) ramp-up Segment Structures Machinery and equipment Bauxite mining 35 17 Alumina refining 30 28 Aluminum smelting and casting 36 22 Energy generation 33 25 Aluminum rolling 31 17 Repairs and maintenance are charged to expense as incurred. Gains or losses from the sale of assets are generally recorded in Other (income) expenses, net. Properties, plants, and equipment are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of such assets (asset group) may not be recoverable. Recoverability of assets is determined by comparing the estimated undiscounted net cash flows of the operations related to the assets (asset group) to their carrying amount. An impairment loss would be recognized when the carrying amount of the assets (asset group) exceeds the estimated undiscounted net cash flows. The amount of the impairment loss to be recorded is calculated as the excess of the carrying value of the assets (asset group) over their fair value, with fair value determined using the best information available, which generally is a discounted cash flow (DCF) model. The determination of what constitutes an asset group, the associated estimated undiscounted net cash flows, and the estimated useful lives of assets also require significant judgments. Equity Investments. Deferred Mining Costs. Goodwill and Other Intangible Assets. Goodwill is allocated among and evaluated for impairment at the reporting unit level, which is defined as an operating segment or one level below an operating segment. Alcoa Corporation has five reporting units, of which three are included in the Aluminum segment (smelting/casting, energy generation, and rolling operations). The remaining two reporting units are the Bauxite and Alumina segments. Of these five reporting units, only Bauxite and Alumina contain goodwill. As of December 31, 2017, the carrying value of the goodwill for Bauxite and Alumina was $51 and $103, respectively. These amounts include an allocation of goodwill held at the corporate level (see Note K). Prior to 2017, the Company had six reporting units equivalent to its then six operating segments as follows: Bauxite, Alumina, Aluminum, Cast Products, Energy, and Rolled Products. In early 2017, management initiated a realignment of the Company’s internal business and organizational structure resulting in a change to Alcoa Corporation’s operating segments and reporting units (see Note E). Of the previous six reporting units, only Bauxite and Alumina contained goodwill. In reviewing goodwill for impairment, an entity has the option to first assess qualitative factors to determine whether the existence of events or circumstances leads to a determination that it is more likely than not (greater than 50%) that the estimated fair value of a reporting unit is less than its carrying amount. If an entity elects to perform a qualitative assessment and determines that an impairment is more likely than not, the entity is then required to perform the existing two-step two-step two-step Alcoa Corporation’s policy for its annual review of goodwill is to perform the qualitative assessment for all reporting units not subjected directly to the two-step two-step Under the qualitative assessment, various events and circumstances (or factors) that would affect the estimated fair value of a reporting unit are identified (similar to impairment indicators above). These factors are then classified by the type of impact they would have on the estimated fair value using positive, neutral, and adverse categories based on current business conditions. Additionally, an assessment of the level of impact that a particular factor would have on the estimated fair value is determined using high, medium, and low weighting. Furthermore, management considers the results of the most recent two-step During the 2017 annual review of goodwill, management performed the qualitative assessment for the Bauxite and Alumina reporting units. Management concluded it was not more likely than not that the respective estimated fair value of these reporting units was less than the respective carrying value. As such, no further analysis was required. Under the two-step In the event the estimated fair value of a reporting unit per the DCF model is less than the carrying value, additional analysis would be required. The additional analysis would compare the carrying amount of the reporting unit’s goodwill with the implied fair value of that goodwill, which may involve the use of valuation experts. The implied fair value of goodwill is the excess of the fair value of the reporting unit over the fair value amounts assigned to all of the assets and liabilities of that unit as if the reporting unit was acquired in a business combination and the fair value of the reporting unit represented the purchase price. If the carrying value of goodwill exceeds its implied fair value, an impairment loss equal to such excess would be recognized. Management last proceeded directly to the two-step Intangible assets with finite useful lives are amortized generally on a straight-line basis over the periods benefited. The following table details the weighted-average useful lives of software and other intangible assets by type of operation (numbers in years): Segment Software Other intangible assets Bauxite mining 9 10 Alumina refining 6 20 Aluminum smelting and casting 5 36 Energy generation 3 29 Aluminum rolling 4 20 Asset Retirement Obligations. Certain conditional asset retirement obligations (CAROs) related to alumina refineries, aluminum smelters, rolling mills, and energy generation facilities have not been recorded in the Consolidated Financial Statements due to uncertainties surrounding the ultimate settlement date. A CARO is a legal obligation to perform an asset retirement activity in which the timing and/or method of settlement are conditional on a future event that may or may not be within Alcoa Corporation’s control. Such uncertainties exist as a result of the perpetual nature of the structures, maintenance and upgrade programs, and other factors. At the date a reasonable estimate of the ultimate settlement date can be made (e.g., planned demolition), Alcoa Corporation would record an ARO for the removal, treatment, transportation, storage, and/or disposal of various regulated assets and hazardous materials such as asbestos, underground and aboveground storage tanks, polychlorinated biphenyls (PCBs), various process residuals, solid wastes, electronic equipment waste, and various other materials. Such amounts may be material to the Consolidated Financial Statements in the period in which they are recorded. Environmental Matters. Litigation Matters. Revenue Recognition. Stock-Based Compensation. Compensation expense for employee equity grants is recognized using the non-substantive Most plan participants can choose whether to receive their award in the form of stock options, stock awards, or a combination of both. This choice is made before the grant is issued and is irrevocable. Pension and Other Postretirement Benefit Plans. non-Alcoa Prior to the Separation Date, certain other plans that were entirely attributable to employees of Alcoa Corporation-related operations (the “Direct Plans”) were accounted for as defined benefit pension and other postretirement benefit plans. Accordingly, the funded and unfunded position of each Direct Plan was recorded in the Consolidated Balance Sheet. Actuarial gains and losses that had not yet been recognized through earnings were recorded in accumulated other comprehensive income, net of taxes, until they were amortized as a component of net periodic benefit cost. The determination of benefit obligations and recognition of expenses related to the Direct Plans is dependent on various assumptions. The major assumptions primarily relate to discount rates, long-term expected rates of return on plan assets, and future compensation increases. ParentCo’s management developed each assumption using relevant company experience in conjunction with market-related data for each individual location in which such plans exist. In preparation for the Separation Transaction, effective August 1, 2016, certain of the Shared Plans were separated into standalone plans for both Alcoa Corporation and ParentCo (see Note N). Additionally, certain of the other remaining Shared Plans were assumed by Alcoa Corporation (See Note N). Accordingly, beginning on August 1, 2016 and forward, the standalone plans and assumed plans were accounted for as defined benefit pension and other postretirement plans. Additionally, the Direct Plans continued to be accounted for as defined benefit pension and other postretirement plans. Derivatives and Hedging. Alcoa Corporation accounts for hedges of firm customer commitments for aluminum as fair value hedges. As a result, the fair values of the derivatives and changes in the fair values of the underlying hedged items are reported as assets and liabilities in the Consolidated Balance Sheet. Changes in the fair values of these derivatives and underlying hedged items generally offset and are recorded each period in Sales, consistent with the underlying hedged item. Alcoa Corporation accounts for hedges of foreign currency exposures and certain forecasted transactions as cash flow hedges. The fair values of the derivatives are recorded as assets and liabilities in the Consolidated Balance Sheet. The effective portions of the changes in the fair values of these derivatives are recorded in Other comprehensive (loss) income and are reclassified to Sales, Cost of goods sold, or Other (income) expenses, net in the period in which earnings are impacted by the hedged items or in the period that the transaction no longer qualifies as a cash flow hedge. These contracts cover the same periods as known or expected exposures, generally not exceeding five years. If no hedging relationship is designated, the derivative is marked to market through Other (income) expenses, net. Cash flows from derivatives are recognized in the Statement of Consolidated Cash Flows in a manner consistent with the underlying transactions. Income Taxes. In all periods prior to the Separation Date, Alcoa Corporation’s operations were included in the income tax filings of ParentCo. The provision for income taxes in Alcoa Corporation’s Statement of Consolidated Operations was determined in the same manner described above, but on a separate return methodology as if the Company was a standalone taxpayer filing hypothetical income tax returns where applicable. Any additional accrued tax liability or refund arising as a result of this approach was assumed to be immediately settled with ParentCo as a component of Parent Company net investment. Deferred tax assets were also determined in the same manner described above and were reflected in the Consolidated Balance Sheet for net operating losses, credits or other attributes to the extent that such attributes were expected to transfer to Alcoa Corporation upon the Separation Transaction. Any difference from attributes generated in a hypothetical return on a separate return basis was adjusted as a component of Parent Company net investment. Valuation allowances are recorded to reduce deferred tax assets when it is more likely than not (greater than 50%) that a tax benefit will not be realized. In evaluating the need for a valuation allowance, management considers all potential sources of taxable income, including income available in carryback periods, future reversals of taxable temporary differences, projections of taxable income, and income from tax planning strategies, as well as all available positive and negative evidence. Positive evidence includes factors such as a history of profitable operations, projections of future profitability within the carryforward period, including from tax planning strategies, and Alcoa Corporation’s experience with similar operations. Existing favorable contracts and the ability to sell products into established markets are additional positive evidence. Negative evidence includes items such as cumulative losses, projections of future losses, or carryforward periods that are not long enough to allow for the utilization of a deferred tax asset based on existing projections of income. Deferred tax assets for which no valuation allowance is recorded may not be realized upon changes in facts and circumstances, resulting in a future charge to establish a valuation allowance. Existing valuation allowances are re-examined re-measured Tax benefits related to uncertain tax positions taken or expected to be taken on a tax return are recorded when such benefits meet a more likely than not threshold. Otherwise, these tax benefits are recorded when a tax position has been effectively settled, which means that the statute of limitation has expired or the appropriate taxing authority has completed their examination even though the statute of limitations remains open. Interest and penalties related to uncertain tax positions are recognized as part of the provision for income taxes and are accrued beginning in the period that such interest and penalties would be applicable under relevant tax law until such time that the related tax benefits are recognized. Foreign Currency. Recently Adopted Accounting Guidance. (last-in, first-out) non-LIFO On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to derivative instruments designated as hedging instruments. These changes clarify that a change in the counterparty to a derivative instrument that has been designated as a hedging instrument does not, in and of itself, require de-designation On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to equity method investments. These changes eliminate the requirement for an investor to adjust an equity method investment, results of operations, and retained earnings retroactively on a step-by-step available-for-sale On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to employee share-based payment accounting. Prior to these changes, an entity must determine for each share-based payment award whether the difference between the deduction for tax purposes and the compensation cost recognized for financial reporting purposes results in either an excess tax benefit or a tax deficiency. Excess tax benefits are recognized in additional paid-in tax-withholding tax-withholding On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to consolidation accounting. Prior to these changes, an entity was required to consider indirect economic interests in a variable interest entity held through related parties under common control as direct interests in their entirety in the entity’s assessment of whether it is the primary beneficiary of the variable interest entity. The changes result in an entity considering such indirect economic interests only on a proportionate basis as indirect interests instead of as direct interests in their entirety. The adoption of these changes had no impact on the Consolidated Financial Statements; however, this guidance will need to be considered in future assessments of whether Alcoa Corporation is the primary beneficiary of a variable interest entity. Recently Issued Accounting Guidance. In January 2017, the FASB issued changes to the assessment of goodwill for impairment as it relates to the quantitative test. Currently, there are two steps when performing a quantitative impairment test. The first step requires an entity to compare the current fair value of a reporting unit to its carrying value. In the event the reporting unit’s estimated fair value is less than its carrying value, an entity performs the second step, which is to compare the carrying amount of the reporting unit’s goodwill with the implied fair value of that goodwill. The implied fair value of goodwill is the excess of the fair value of the reporting unit over the fair value amounts assigned to all of the assets and liabilities of that unit as if the reporting unit was acquired in a business combination and the fair value of the reporting unit represented the purchase price. If the carrying value of goodwill exceeds its implied fair value, an impairment loss equal to such excess would be recognized. These changes eliminate the second step of the quantitative impairment test. Accordingly, an entity would recognize an impairment of goodwill for a reporting unit, if under what is currently referred to as the first step, the estimated fair value of the reporting unit is less than the carrying value. The impairment would be equal to the excess of the reporting unit’s carrying value over its fair value not to exceed the total amount of goodwill applicable to that reporting unit. These changes become effective for Alcoa Corporation on January 1, 2020. Management has determined that the adoption of these changes will not have an immediate impact on the Consolidated Financial Statements. This guidance will need to be considered each time Alcoa Corporation performs an assessment of goodwill for impairment under the quantitative test. In March 2017, the FASB issued changes to the presentation of net periodic benefit cost related to pension and other postretirement benefit plans. These changes require that an entity report the service cost component of net periodic benefit cost in the same line item(s) on the statement of operations as other compensation costs arising from services rendered by the pertinent employees during a reporting period. The other components of net periodic benefit cost (see Note N) are required to be presented separately from the service cost component. In other words, these other components may be aggregated and presented as a separate line item or they may be included in existing line items on the statement of operations other than such line items that include the service cost component. Currently, Alcoa Corporation includes all components of net periodic benefit cost, except for certain settlements, curtailments, and special termination benefits related to severance programs, in Cost of goods sold (business employees) and Selling, general administrative, and other expenses (corporate employees) consistent with the location of other compensation costs related to the respective employees. The non-service non-service non-service In May 2017, the FASB issued changes to the accounting for stock-based compensation when there has been a modification to the terms or conditions of a share-based payment award. These changes require an entity to account for the modification only when there has been a substantive change to the terms or conditions of a share-based payment award. A substantive change occurs when the fair value, vesting conditions or balance sheet classification (liability or equity) of a share-based payment award is/are different immediately before and after the modification. Currently, an entity is required to account for any modification in the terms or conditions of a share-based payment award. These changes become effective for Alcoa Corporation on January 1, 2018. Management has determined that the adoption of these changes will not have an immediate impact on the Consolidated Financial Statements. Additionally, the Company will no longer account for any future non-substantive In August 2017, the FASB issued changes to the accounting for hedging activities. These changes permit hedge accounting for risk components in hedging relationships involving nonfinancial risk and interest rate risk; reduce current limitations on the designation and measurement of a hedged item in a fair value hedge of interest rate risk; remove the requirement to separately measure and report hedge ineffectiveness; provide an election to systematically and rationally recognize in earnings the initial value of any amount excluded from the assessment of hedge effectiveness for all types of hedges; and ease the requirements of effectiveness testing. Additionally, modifications to existing disclosures, as well as additional disclosures, will be required to reflect these changes regarding the measurement and recording of hedging activities. These changes become effective for Alcoa Corporation on January 1, 2019 (early adoption is permitted). Management is currently evaluating early adopting these changes and the potential impact of these changes on the Consolidated Financial Statements. In January 2016, the FASB issued changes to the accounting and reporting of certain equity investments. These changes require equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) to be measured at fair value with changes in fair value recognized in net income. However, an entity may choose to measure equity investments that do not have readily determinable fair values at cost minus impairment, if any, plus or minus changes resulting from observable price changes in orderly transactions for the identical or similar investment of the same issuer. Additionally, the impairment assessment of equity investments without readily determinable fair values has been simplified by requiring a qualitative assessment to identify impairment. These changes become effective for Alcoa Corporation on January 1, 2018. Management has determined that the adoption of these changes will not have an impact on the Consolidated Financial Statements, as all of Alcoa Corporation’s equity investments are accounted for under the equity method of accounting. In February 2016, the FASB issued changes to the accounting and presentation of leases. These changes require lessees to recognize a right of use asset and lease liability on the balance sheet for all leases with terms longer than 12 months. For leases with a term of 12 months or less, a lessee is permitted to make an accounting policy election by class of underlying asset not to recognize a right of use asset and lease liability. Additionally, when measuring assets and liabilities arising from a lease, optional payments should be included only if the lessee is reasonably certain to exercise an option to extend the lease, exercise a purchase option, or not exercise an option to terminate the lease. These changes become effective for Alcoa Corporation on January 1, 2019. The Company has established a cross-functional project team to lead the implementation effort. This team has determined that the Company requires information systems updates and incremental software to effectively implement these changes. Accordingly, the Company has selected a software vendor and is in the early stages of implementing lease management software. Upon adoption of these changes, management does expect to record a right of use asset and lease liability on Alcoa Corporation’s Consolidated Balance Sheet. While the precise amount of this asset and liability will not be known until closer to the adoption date, management estimates the amount to be less than 5% of both total assets and total liabilities. This estimate is based on Alcoa Corporation’s Consolidated Balance Sheet and lease portfolio, both as of December 31, 2017. In June 2016, the FASB added a new impairment model (known as the current expected credit loss (CECL) model) that is based on expected losses rather than incurred losses. Under the new guidance, an entity recognizes as an allowance its estimate of expected credit losses. The CECL model applies to most debt instruments, trade receivables, lease receivables, financial guarantee contracts, and other loan commitments. The CECL model does not have a minimum threshold for recognition of impairment losses and entities will need to measure expected credit losses on assets that have a low risk of loss. These changes become effective for Alcoa Corporation on January 1, 2020. Management is currently evaluating the potential impact of these changes on the Consolidated Financial Statements. In both August and November 2016, the FASB issued changes to the presentation of a number of items in the statement of cash flows. Specifically, the changes identify nine specific cash flow items and the sections where they must be presented within the statement of cash flows, including distributions received from equity method investees, proceeds from the settlement of insurance claims, and restricted cash. These changes become effective for Alcoa Corporation on January 1, 2018. Management has determined that the adoption of these changes will not have a material impact on the Consolidated Financial Statements. In October 2016, the FASB issued changes to the accounting for intra-entity transactions, other than inventory. Currently, no immediate tax impact is recognized in an entity’s financial statements as a result of intra-entity transfers of assets. An entity is precluded from reflecting a tax benefit or expense from an intra-entity asset transfer between entities that file separate tax returns, whether or not such entities are in different tax jurisdictions, until the asset has been sold to a third party or otherwise recovered. The buyer of such asset is prohibited from recognizing a deferred tax asset for the temporary difference arising from the excess of the buyer’s tax basis over the cost to the seller. The changes require the current and deferred income tax consequences of the intra-entity transfer to be recorded when the transaction occurs. The exception to defer the tax consequences of inventory transactions is maintained. These changes become effective for Alcoa Corporation on January 1, 2018. Management has determined that the adoption of these changes will not have a material impact on the Consolidated Financial Statements. In May 2014, the FASB issued changes to the recognition of revenue from contracts with customers. These changes created a comprehensive framework for all entities in all industries to apply in the determination of when to recognize revenue, and, therefore, supersede virtually all existing revenue recognition requirements and guidance. This framework is expected to result in less complex guidance in application while providing a consistent and comparable methodology for revenue recognition. The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To achieve this principle, an entity should apply the following steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract(s), (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract(s), and (v) recognize revenue when, or as, the entity satisfies a performance obligation. In August 2015, the FASB deferred the effective date by one year, making these changes effective for Alcoa Corporation on January 1, 2018. Through a previously established project team, the Company completed a detailed review of the terms and provisions of its customer contracts by mid-2017.

Acquisitions and Divestitures

Acquisitions and Divestitures	12 Months Ended
	Dec. 31, 2017
Business Combinations [Abstract]
Acquisitions and Divestitures	C. Acquisitions and Divestitures In February 2017, Alcoa Corporation’s wholly-owned subsidiary, Alcoa Power Generating Inc., completed the sale of its 215-megawatt (pre- after-tax) 38-mile In December 2014, Alcoa Corporation’s majority-owned subsidiary (60%), Alcoa Minerals of Jamaica, LLC (part of AWAC), completed the sale of its 55% ownership stake in a bauxite mine and alumina refinery joint venture in Jamaica to Noble Group Ltd. Also in December 2014, Alcoa Corporation completed the sale of its 50.33% ownership stake in the Mt. Holly smelter located in Goose Creek, South Carolina to Century Aluminum Company. Combined, these transactions yielded net cash proceeds of $185 and resulted in a net loss of $240, which was recorded in Restructuring and other charges on the 2014 Statement of Consolidated Operations. In 2015, Alcoa Corporation had post-closing adjustments, as provided for in the respective purchase agreements, related to these two divestitures. The combined post-closing adjustments resulted in net cash received of $41 and a net loss of $24, which was recorded in Restructuring and other charges (see Note D) on the accompanying Statement of Consolidated Operations. These two divestitures are no longer subject to post-closing adjustments.

Restructuring and Other Charges

Restructuring and Other Charges	12 Months Ended
	Dec. 31, 2017
Restructuring and Related Activities [Abstract]
Restructuring and Other Charges	D. Restructuring and Other Charges Restructuring and other charges for each year in the three-year period ended December 31, 2017 were comprised of the following: 2017 2016 2015 Early termination of a power contract $ 244 $ - $ - Asset impairments 40 155 311 Layoff costs 23 32 199 Legal matters in Italy (R) (22 ) - 201 Asset retirement obligations (Q) 10 97 76 Environmental remediation (R) 8 26 86 Net (gain) loss on divestitures of businesses (C) - (3 ) 25 Other 49 47 92 Reversals of previously recorded layoff and other costs (43 ) (36 ) (7 ) Restructuring and other charges $ 309 $ 318 $ 983 * In 2016 and 2015, Other includes $1 and $32, respectively, related to the allocation of restructuring charges to Alcoa Corporation from ParentCo (see Note A). Layoff costs were recorded based on approved detailed action plans submitted by the operating locations that specified positions to be eliminated, benefits to be paid under existing severance plans, union contracts or statutory requirements, and the expected timetable for completion of the plans. 2017 Actions. In October 2017, Alcoa Corporation and Luminant Generation Company LLC (Luminant) executed an early termination agreement of a power contract, as well as other related fuel and lease agreements, effective October 1, 2017, related to the Company’s Rockdale (Texas) smelter, which has been fully curtailed since the end of 2008. In accordance with the terms of the early termination agreement, Alcoa made a cash payment of $238 and transferred approximately 2,200 acres of related land and other assets and liabilities to Luminant (net asset carrying value of $6). Since the curtailment of the Rockdale smelter, the Company had been selling surplus electricity into the energy market. The power contract was set to expire no earlier than 2038, except for limited circumstances in which one or both parties could elect to early terminate without penalty for which conditions had never been met. In 2017 (through September 30), 2016, and 2015, Alcoa Corporation recognized $105, $141, and $147, respectively, in Sales and $148, $210, and $201, respectively, in Cost of goods sold on the accompanying Statement of Consolidated Operations related to the sale of the surplus electricity and the cost of the Luminant power contract. As a result of the early termination of the power contract, Alcoa initiated a strategic review of the remaining buildings and equipment associated with the smelter, casthouse, and the aluminum powder plant at the Rockdale location. ParentCo previously decided to curtail the operating capacity of the Rockdale smelter in 2008 as a result of an uncompetitive power supply and then-overall unfavorable market conditions. Under this review, which was completed in December 2017, management determined that the Rockdale operations have limited economic prospects. Consequently, management approved the permanent closure and demolition of the Rockdale smelter (capacity of 191 kmt-per-year) In 2017, costs related to this decision included asset impairments of $32, representing the write-off In July 2017, Alcoa Corporation announced plans to restart three (161,400 metric tons of capacity) of the five potlines (268,800 metric tons of capacity) at the Warrick (Indiana) smelter, which is expected to be complete in the second quarter of 2018. This smelter was previously permanently closed in March 2016 by ParentCo (see 2016 Actions below). The capacity identified for restart will directly supply the existing rolling mill at the Warrick location to improve efficiency of the integrated site and provide an additional source of metal to help meet an anticipated increase in production volumes. As a result of the decision to reopen this smelter, in 2017, Alcoa Corporation reversed $33 in remaining liabilities related to the original closure decision. These liabilities consisted of $20 in asset retirement obligations and $4 in environmental remediation obligations, which were necessary due to the previous decision to demolish the smelter, and $9 in severance and contract termination costs. Additionally, the carrying value of the smelter and related assets was reduced to zero as the smelter ramped down between the permanent closure decision date (end of 2015) and the end of March 2016. Once these assets are placed back into service in conjunction with the restart, their carrying value will remain zero. As such, only newly acquired or constructed assets related to the Warrick smelter will be depreciated. As of December 31, 2017, approximately 90 of the 140 employees were separated. The remaining separations for 2017 restructuring programs are expected to be completed by the end of 2018. In 2017, cash payments of $9 were made against layoff reserves related to 2017 restructuring programs. 2016 Actions. In December 2016, management approved the permanent closure of the Suralco refinery (capacity of 2,207 kmt-per-year) In 2016, costs related to the closure and curtailment actions included accelerated depreciation of $70 related to the Warrick smelter as it continued to operate through March 2016; asset impairments of $16, representing the write-off Also in December 2016, management of Alcoa Corporation concluded that an interest in certain gas exploration assets in Western Australia has been impaired. AofA owns an interest in a gas exploration project that was initially entered into in 2007 as a potential source of low-cost re-evaluated As of June 30, 2017, the separations associated with 2016 restructuring programs were essentially complete. In 2017 and 2016, cash payments of $2 and $7, respectively, were made against layoff reserves related to 2016 restructuring programs. 2015 Actions. During 2015, management initiated various alumina refining and aluminum smelting capacity curtailments and/or closures. The curtailments were composed of the remaining capacity at all of the following: the São Luís smelter in Brazil (74 kmt-per-year); kmt-per-year); kmt-per-year); kmt-per-year). kmt-per-year kmt-per-year) The decisions on the above actions were part of a separate 12-month kmt-per-year) kmt-per-year) Separate from the actions initiated under the reviews described above, in mid-2015, kmt-per-year) The decision on the Poços de Caldas smelter was due to management’s conclusion that the smelter was no longer competitive as a result of challenging global market conditions for primary aluminum, which led to the initial curtailment, that have not dissipated and higher costs. For the Anglesea power station, the decision was made because a sale process did not result in a sale and there would have been imminent operating costs and financial constraints related to this site in the remainder of 2015 and beyond, including significant costs to source coal from available resources, necessary maintenance costs, and a depressed outlook for forward electricity prices. The Anglesea power station previously supplied approximately 40 percent of the power needs for the Point Henry smelter, which was closed in August 2014. In 2015, costs related to the closure and curtailment actions included asset impairments of $226, representing the write-off As of June 30, 2017, the separations associated with 2015 restructuring programs were essentially complete. In 2017, 2016, and 2015, cash payments of $18, $65, and $26, respectively, were made against layoff reserves related to 2015 restructuring programs. Alcoa Corporation does not include Restructuring and other charges in the results of its reportable segments. The impact of allocating such charges to segment results would have been as follows: 2017 2016 2015 Bauxite $ 2 $ (5 ) $ 8 Alumina 3 72 102 Aluminum 51 75 100 Segment total 56 142 210 Corporate 253 176 773 Total restructuring and other charges $ 309 $ 318 $ 983 Activity and reserve balances for restructuring charges were as follows: Layoff Other Total Reserve balances at December 31, 2014 $ 50 $ 13 $ 63 2015: Cash payments (65 ) (1 ) (66 ) Restructuring charges 199 222 421 Other* (47 ) (219 ) (266 ) Reserve balances at December 31, 2015 137 15 152 2016: Cash payments (74 ) (35 ) (109 ) Restructuring charges 32 168 200 Other* (57 ) (120 ) (177 ) Reserve balances at December 31, 2016 38 28 66 2017: Cash payments (30 ) (43 ) (73 ) Restructuring charges 23 67 90 Other* (20 ) (18 ) (38 ) Reserve balances at December 31, 2017 $ 11 $ 34 $ 45 * Other includes reversals of previously recorded restructuring charges and the effects of foreign currency translation. In 2017, 2016, and 2015, Other for Layoff costs also included a reclassification of $8, $16, and $35, respectively, in pension and/or other postretirement benefits costs, as these obligations were included in Alcoa Corporation’s separate liability for pension and other postretirement benefits obligations (see Note N). Additionally in 2017, 2016, and 2015, Other for Other costs also included a reclassification of the following restructuring charges: $10, $97, and $76, respectively, in asset retirement and $8, $26, and $86, respectively, in environmental obligations, as these liabilities were included in Alcoa Corporation’s separate reserves for asset retirement obligations (see Note Q) and environmental remediation (see Note R). The remaining reserves are expected to be paid in cash during 2018, with the exception of $4, which relates to the termination of an office lease contract and is expected to be paid by no later than the end of 2020.

Segment and Geographic Area Inf

Segment and Geographic Area Information	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Segment and Geographic Area Information	E. Segment and Geographic Area Information Segment Information Alcoa Corporation is a producer of bauxite, alumina, and aluminum products (primary and flat-rolled). Segment performance under the Company’s management reporting system is evaluated based on a number of factors; however, the primary measure of performance is the Adjusted EBITDA (Earnings before interest, taxes, depreciation, and amortization) (see below) of each segment. Segment assets include, among others, customer receivables (third-party and intersegment), inventories (excluding LIFO adjustments), properties, plants, and equipment, and equity investments. The accounting policies of the segments are the same as those described in the Summary of Significant Accounting Policies (see Note B). Transactions among segments are established based on negotiation among the parties. Differences between segment totals and Alcoa Corporation’s consolidated totals for line items not reconciled are in Corporate. Effective in the first quarter of 2017, management elected to change the profit and loss measure of Alcoa Corporation’s reportable segments from After-tax Also effective in the first quarter of 2017, management initiated a realignment of the Company’s internal business and organizational structure. This realignment consisted of combining Alcoa Corporation’s aluminum smelting, casting, and rolling businesses, along with the majority of the energy business, into a new Aluminum business unit, as well as moving the financial results of previously closed operations, such as the Warrick smelter and Suriname refinery, into Corporate. The realignment was executed to align strategic, operational, and commercial activities, as well as to take advantage of synergies and reduce costs. The new Aluminum business unit is managed as a single operating segment. Prior to this change, each of these businesses were managed as individual operating segments and comprised the Aluminum, Cast Products, Energy, and Rolled Products segments. The existing Bauxite and Alumina segments and the new Aluminum segment represent Alcoa Corporation’s operating and reportable segments. The chief operating decision maker function regularly reviews the financial information, including Sales and Adjusted EBITDA, of these three operating segments to assess performance and allocate resources. Segment information for all prior periods presented was revised to reflect the new segment structure, as well as the new measure of profit and loss. The following are detailed descriptions of Alcoa Corporation’s reportable segments: Bauxite. Alumina. Australian dollar, the Brazilian real, the U.S. dollar, and the euro. Most of the operations that comprise the Alumina segment are part of AWAC (see Principles of Consolidation in Note A). This segment also includes AWAC’s 25.1% share of the results of a mining and refining joint venture company in Saudi Arabia (see Note H). Aluminum. value-add The operating results, capital expenditures, and assets of Alcoa Corporation’s reportable segments were as follows: Bauxite Alumina Aluminum Total 2017 Sales: Third-party sales—unrelated party $ 333 $ 3,133 $ 7,163 $ 10,629 Third-party sales—related party - - 864 864 Intersegment sales 875 1,723 21 2,619 Total sales $ 1,208 $ 4,856 $ 8,048 $ 14,112 Adjusted EBITDA $ 427 $ 1,289 $ 964 $ 2,680 Supplemental information: Depreciation, depletion, and amortization $ 82 $ 207 $ 419 $ 708 Equity loss - (5 ) (19 ) (24 ) 2016 Sales: Third-party sales—unrelated party $ 315 $ 2,300 $ 5,573 $ 8,188 Third-party sales—related party - - 958 958 Intersegment sales 751 1,307 42 2,100 Total sales $ 1,066 $ 3,607 $ 6,573 $ 11,246 Adjusted EBITDA $ 375 $ 378 $ 680 $ 1,433 Supplemental information: Depreciation, depletion, and amortization $ 77 $ 186 $ 414 $ 677 Equity loss - (40 ) (24 ) (64 ) 2015 Sales: Third-party sales—unrelated party $ 71 $ 3,341 $ 6,479 $ 9,891 Third-party sales—related party - - 1,078 1,078 Intersegment sales 1,076 1,727 175 2,978 Total sales $ 1,147 $ 5,068 $ 7,732 $ 13,947 Adjusted EBITDA $ 454 $ 958 $ 767 $ 2,179 Supplemental information: Depreciation, depletion, and amortization $ 93 $ 192 $ 424 $ 709 Equity loss - (41 ) (44 ) (85 ) 2017 Assets: Capital expenditures $ 53 $ 144 $ 178 $ 375 Equity investments 191 262 930 1,383 Total assets 1,609 5,129 8,060 14,798 2016 Assets: Capital expenditures $ 29 $ 109 $ 256 $ 394 Equity investments 163 342 859 1,364 Total assets 1,541 4,791 7,658 13,990 The following tables reconcile certain segment information to consolidated totals: 2017 2016 2015 Sales: Total segment sales $ 14,112 $ 11,246 $ 13,947 Elimination of intersegment sales (2,619 ) (2,100 ) (2,978 ) Other 159 172 230 Consolidated sales $ 11,652 $ 9,318 $ 11,199 2017 2016 2015 Net income (loss) attributable to Alcoa Corporation: Total segment Adjusted EBITDA $ 2,680 $ 1,433 $ 2,179 Unallocated amounts: Impact of LIFO (I) (91 ) (10 ) 107 Metal price lag (1) 26 9 (30 ) Corporate expense (2) (136 ) (177 ) (173 ) Provision for depreciation, depletion, and amortization (750 ) (718 ) (780 ) Restructuring and other charges (D) (309 ) (318 ) (983 ) Interest expense (S) (104 ) (243 ) (270 ) Other income (expenses), net (S) 58 89 (42 ) Other (3) (215 ) (227 ) (345 ) Consolidated income (loss) before income taxes 1,159 (162 ) (337 ) Provision for income taxes (P) (600 ) (184 ) (402 ) Net income attributable to noncontrolling interest (342 ) (54 ) (124 ) Consolidated net income (loss) attributable to Alcoa Corporation $ 217 $ (400 ) $ (863 ) (1) Metal price lag describes the timing difference created when the average price of metal sold differs from the average cost of the metal when purchased by Alcoa Corporation’s rolled aluminum operations. In general, when the price of metal increases, metal price lag is favorable, and when the price of metal decreases, metal price lag is unfavorable. (2) Corporate expense is primarily composed of general administrative and other expenses of operating the corporate headquarters and other global administrative facilities. (3) Other includes, among other items, the Adjusted EBITDA of previously closed operations as applicable, pension and other postretirement benefit expenses associated with closed and sold operations, and intersegment profit elimination. December 31, 2017 2016 Assets: Total segment assets $ 14,798 $ 13,990 Elimination of intersegment receivables (299 ) (236 ) Unallocated amounts: Cash and cash equivalents 1,358 853 LIFO reserve (306 ) (215 ) Corporate fixed assets, net 520 595 Corporate goodwill 148 149 Deferred income taxes 814 741 Fair value of derivative contracts 34 497 Other 380 367 Consolidated assets $ 17,447 $ 16,741 Sales by major product grouping were as follows: 2017 2016 2015 Sales: Primary aluminum $ 6,168 $ 5,204 $ 6,214 Alumina 3,121 2,280 3,325 Flat-rolled aluminum 1,666 1,068 989 Energy 446 422 588 Bauxite 333 315 71 Other* (82 ) 29 12 $ 11,652 $ 9,318 $ 11,199 * Other includes realized gains and losses related to embedded derivative instruments designated as cash flow hedges of forward sales of aluminum (see Note O). Geographic Area Information Geographic information for sales was as follows (based upon the country where the point of sale originated): 2017 2016 2015 Sales: United States (1) $ 5,370 $ 4,365 $ 5,386 Spain (2) 3,303 2,663 2,852 Australia 2,266 1,644 2,147 Brazil 569 432 562 Canada 93 141 132 Other 51 73 120 $11,652 $9,318 $11,199 (1) Sales of a portion of the alumina from refineries in Australia, Brazil, and Suriname (prior to closure in December 2016) and most of the aluminum from smelters in Canada occurred in the United States. (2) Sales of the aluminum produced from smelters in Iceland and Norway, as well as the off-take Geographic information for long-lived assets was as follows (based upon the physical location of the assets): December 31, 2017 2016 Long-lived assets: Australia $ 2,220 $ 2,053 Brazil 2,111 2,228 United States 1,658 1,816 Iceland 1,276 1,341 Canada 1,116 1,161 Norway 427 438 Spain 316 273 Other 14 15 $9,138 $9,325

Earnings Per Share

Earnings Per Share	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Earnings Per Share	F. Earnings Per Share Basic earnings per share (EPS) amounts are computed by dividing earnings by the average number of common shares outstanding. Diluted EPS amounts assume the issuance of common stock for all potentially dilutive share equivalents outstanding. The information used to compute basic and diluted EPS attributable to Alcoa Corporation common shareholders was as follows (shares in millions): 2017 2016 2015 Net income (loss) attributable to Alcoa Corporation $ 217 $ (400 ) $ (863 ) Average shares outstanding—basic 184 183 182 Effect of dilutive securities: Stock options 1 - - Stock and performance awards 2 - - Average shares outstanding—diluted 187 183 182 In 2016, basic average shares outstanding and diluted average shares outstanding were the same because the effect of potential shares of common stock was anti-dilutive. Options to purchase 1 million shares of common stock outstanding as of December 31, 2016 at a weighted average exercise price of $33.05 per share were not included in the computation of diluted EPS because the exercise prices of these options were greater than the average market price of Alcoa Corporation’s common stock. Additionally, 1 million stock awards and stock options combined were not included in the computation of diluted EPS because Alcoa Corporation generated a net loss. Had Alcoa Corporation generated net income in 2016, 1 million potential shares of common stock related to stock awards and stock options combined would have been included in diluted average shares outstanding. In 2015, the EPS included on the accompanying Statement of Consolidated Operations was calculated based on the 182,471,195 shares of Alcoa Corporation common stock distributed on the Separation Date in conjunction with the completion of the Separation Transaction and is considered pro forma in nature. Prior to November 1, 2016, Alcoa Corporation did not have any issued and outstanding publicly-traded common stock.

Accumulated Other Comprehensive

Accumulated Other Comprehensive Loss	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Accumulated Other Comprehensive Loss	G. Accumulated Other Comprehensive Loss The following table details the activity of the three components that comprise Accumulated other comprehensive loss for both Alcoa Corporation’s shareholders and noncontrolling interest: Alcoa Corporation Noncontrolling interest 2017 2016 2015 2017 2016 2015 Pension and other postretirement benefits (N) Balance at beginning of period $ (2,330 ) $ (352 ) $ (424 ) $ (56 ) $ (56 ) $ (64 ) Establishment of additional defined benefit plans - (2,704 ) - - - - Separation-related adjustments (A) - 928 - - - - Other comprehensive (loss) income: Unrecognized net actuarial loss and prior service cost/benefit (671 ) (307 ) 73 9 2 5 Tax benefit (expense) 25 6 (18 ) (2 ) (6 ) (1 ) Total Other comprehensive (loss) income before reclassifications, net of tax (646 ) (301 ) 55 7 (4 ) 4 Amortization of net actuarial loss and prior service cost/benefit (1) 199 107 26 2 5 6 Tax expense (2) (9 ) (8 ) (9 ) - (1 ) (2 ) Total amount reclassified from Accumulated other comprehensive loss, net of tax (7) 190 99 17 2 4 4 Total Other comprehensive (loss) income (456 ) (202 ) 72 9 - 8 Balance at end of period $ (2,786 ) $ (2,330 ) $ (352 ) $ (47 ) $ (56 ) $ (56 ) Foreign currency translation Balance at beginning of period $ (1,655 ) $ (1,851 ) $ (668 ) $ (677 ) $ (779 ) $ (351 ) Separation-related adjustments (A) - (17 ) - - - - Other comprehensive income (loss) (3) 188 213 (1,183 ) 96 102 (428 ) Balance at end of period $ (1,467 ) $ (1,655 ) $ (1,851 ) $ (581 ) $ (677 ) $ (779 ) Cash flow hedges (O) Balance at beginning of period $ 210 $ 603 $ (224 ) $ 1 $ (3 ) $ (2 ) Separation-related adjustments (A) - (47 ) - - - - Other comprehensive (loss) income: Net change from periodic revaluations (1,489 ) (558 ) 1,155 83 38 (1 ) Tax benefit (expense) 251 233 (344 ) (25 ) (12 ) - Total Other comprehensive (loss) income before reclassifications, net of tax (1,238 ) (325 ) 811 58 26 (1 ) Net amount reclassified to earnings: Aluminum contracts (4) 130 7 21 - - - Financial contract (5) (19 ) (54 ) - (12 ) (37 ) - Foreign exchange contracts (4) (2 ) - - - - - Interest rate contract (6) - 7 - - 5 - Sub-total 109 (40 ) 21 (12 ) (32 ) - Tax (expense) benefit (2) (10 ) 19 (5 ) 4 10 - Total amount reclassified from Accumulated other comprehensive loss, net of tax (7) 99 (21 ) 16 (8 ) (22 ) - Total Other comprehensive (loss) income (1,139 ) (346 ) 827 50 4 (1 ) Balance at end of period $ (929 ) $ 210 $ 603 $ 51 $ 1 $ (3 ) (1) These amounts were included in the computation of net periodic benefit cost for pension and other postretirement benefits (see Note N). (2) These amounts were included in Provision for income taxes on the accompanying Statement of Consolidated Operations. (3) In all periods presented, there were no tax impacts related to rate changes and no amounts were reclassified to earnings. (4) These amounts were included in Sales on the accompanying Statement of Consolidated Operations. (5) The 2017 amounts were included in Cost of goods sold on the accompanying Statement of Consolidated Operations. The 2016 and 2015 amounts were included in Other (income) expenses, net on the accompanying Statement of Consolidated Operations. (6) These amounts were included in Other (income) expenses, net on the accompanying Statement of Consolidated Operations. (7) A positive amount indicates a corresponding charge to earnings and a negative amount indicates a corresponding benefit to earnings. These amounts were reflected on the accompanying Statement of Consolidated Operations in the line items indicated in footnotes 1 through 6.

Investments

Investments	12 Months Ended
	Dec. 31, 2017
Equity Method Investments and Joint Ventures [Abstract]
Investments	H. Investments December 31, 2017 2016 Equity investments $ 1,400 $ 1,348 Other investments 10 10 $1,410 $1,358 Equity Investments. Investee Country Nature of investment (4) Ownership interest Ma’aden Aluminum Company (1) Saudi Arabia Aluminum smelter 25.1 % Ma’aden Bauxite and Alumina Company (1) Saudi Arabia Bauxite mine and alumina refinery 25.1 % (5) Ma’aden Rolling Company (1) Saudi Arabia Aluminum rolling mill 25.1 % Halco Mining, Inc. (2) Guinea Bauxite mine 45 % (5) Energetica Barra Grande S.A. Brazil Hydroelectric generation facility 42.18 % Pechiney Reynolds Quebec, Inc. (3) Canada Aluminum smelter 50 % Consorcio Serra do Facão Brazil Hydroelectric generation facility 34.97 % Mineração Rio do Norte S.A. Brazil Bauxite mine 18.2 % (5) Manicouagan Power Limited Partnership Canada Hydroelectric generation facility 40 % (1) See Saudi Arabia Joint Venture below for additional information. (2) Halco Mining, Inc. owns 100% of Boké Investment Company, which owns 51% of Compagnie des Bauxites de Guinée. (3) Pechiney Reynolds Quebec, Inc. owns a 50.1% interest in the Bécancour smelter in Quebec, Canada thereby entitling Alcoa Corporation to a 25.05% interest in the smelter. Through two wholly-owned Canadian subsidiaries, Alcoa Corporation also owns 49.9% of the Bécancour smelter. (4) Each of the investees either owns the facility listed or has an ownership interest in an entity that owns the facility listed. (5) A portion or all of each of these ownership interests are held by majority-owned subsidiaries that are part of AWAC. In 2017, 2016, and 2015, Alcoa Corporation received $71, $74, and $152, respectively, in dividends from these equity investments. Financial information for these equity investments is as follows (amounts represent 100% of the investee’s financial information): Saudi Arabia (1) Halco Energetica Pechiney Consorcio Mineração Manicouagan Power L.P. DBNGP (2) Total Profit and loss data—year ended December 31, 2017 Sales $ 3,032 $ 416 $ 131 $ 332 $ 103 $ 350 $ 105 $ - $ 4,469 Cost of goods sold 2,776 266 117 292 48 273 9 - 3,781 (Loss) Income before income taxes (142 ) 50 13 35 45 35 96 - 132 Net (loss) income (157 ) 47 5 23 46 30 88 - 82 Equity in net (loss) income of affiliated companies, before reconciling adjustments (39 ) 21 2 11 16 6 35 - 52 Other 9 (1 ) - - - - 2 - 10 Alcoa Corporation’s equity in net (loss) income of affiliated companies (30 ) 20 2 11 16 6 37 - 62 Profit and loss data—year ended December 31, 2016 Sales $ 1,970 $ 437 $ 60 $ 309 $ 90 $ 451 $ 104 $ 86 $ 3,507 Cost of goods sold 1,905 242 35 272 65 297 10 18 2,844 (Loss) Income before income taxes (295 ) 53 16 36 8 152 94 21 85 Net (loss) income (295 ) 50 15 16 5 129 87 14 21 Equity in net (loss) income of affiliated companies, before reconciling adjustments (75 ) 23 6 8 2 23 35 3 25 Other 7 2 (1 ) (4 ) - (1 ) - - 3 Alcoa Corporation’s equity in net (loss) income of affiliated companies (68 ) 25 5 4 2 22 35 3 28 Profit and loss data—year ended December 31, 2015 Sales $ 2,025 $ 487 $ 130 $ 486 $ 84 $ 498 $ 106 $ 303 $ 4,119 Cost of goods sold 2,070 236 98 288 76 308 16 117 3,209 (Loss) Income before income taxes (362 ) 86 27 113 6 118 91 46 125 Net (loss) income (366 ) 80 7 104 (1 ) 98 90 31 43 Equity in net (loss) income of affiliated companies, before reconciling adjustments (91 ) 36 3 52 - 18 36 6 60 Other (2 ) (2 ) (1 ) 4 (2 ) (3 ) - (1 ) (7 ) Alcoa Corporation’s equity in net (loss) income of affiliated companies (93 ) 34 2 56 (2 ) 15 36 5 53 Balance sheet data—as of December 31, 2017 Current assets $ 1,415 $ 40 $ 44 $ 140 $ 79 $ 121 $ 23 $ - $ 1,862 Noncurrent assets 9,373 174 285 106 261 744 72 - 11,015 Current liabilities 1,331 1 48 65 25 220 9 - 1,699 Noncurrent liabilities 6,191 12 6 12 117 413 - - 6,751 Balance Sheet data—as of December 31, 2016 Current assets $ 1,141 $ 12 $ 17 $ 91 $ 19 $ 92 $ 24 $ - $ 1,396 Noncurrent assets 9,653 189 302 147 451 644 68 - 11,454 Current liabilities 1,452 4 31 62 37 174 7 - 1,767 Noncurrent liabilities 6,204 14 23 - 283 253 - - 6,777 (1) The amounts included in this column represent the combined financial information related to Ma’aden Aluminum Company, Ma’aden Bauxite and Alumina Company, and Ma’aden Rolling Company. (2) AofA sold its interest in the DBNGP Trust in April 2016. Saudi Arabia Joint Venture— 30-year In 2012, Alcoa Corporation and Ma’aden agreed to expand the capabilities of the rolling mill to include a capacity of 100 kmt dedicated to supplying the automotive, building and construction, and foil packaging markets with aluminum sheet. First production related to the expanded capacity occurred in 2014. This expansion did not result in additional equity investment (see below) due to significant savings from a change in the project execution strategy of the initial 380 kmt capacity of the rolling mill. The joint venture is owned 74.9% by Ma’aden and 25.1% by Alcoa Corporation and consists of three separate companies as follows: one each for the mine and refinery, the smelter, and the rolling mill. The Alcoa Corporation affiliates that hold the Company’s interests in the smelting company and the rolling mill company are wholly-owned by Alcoa Corporation, and the Alcoa Corporation affiliate that holds the Company’s interests in the mining and refining company is majority-owned (part of AWAC) by Alcoa Corporation. Except in limited circumstances, Alcoa Corporation may not sell, transfer or otherwise dispose of or encumber or enter into any agreement in respect of the votes or other rights attached to its interests in the joint venture without Ma’aden’s prior written consent. Ma’aden and Alcoa Corporation have put and call options, respectively, whereby Ma’aden can require Alcoa Corporation to purchase from Ma’aden, or Alcoa Corporation can require Ma’aden to sell to Alcoa Corporation, a 14.9% interest in the joint venture at the then fair market value. These options may only be exercised in a six-month Ma’aden and Alcoa Corporation also may not sell, transfer, or otherwise dispose of, pledge, or encumber any interests in the joint venture until five years after the Commercial Production Date. Under the joint venture shareholders’ agreement, upon the occurrence of an unremedied event of default by Alcoa Corporation, Ma’aden may purchase, or, upon the occurrence of an unremedied event of default by Ma’aden, Alcoa Corporation may sell, its interest for consideration that varies depending on the time of the default. A number of Alcoa Corporation employees perform various types of services for the smelting, rolling mill, and mining and refining companies as part of the operation of the fully-integrated aluminum complex. At December 31, 2017 and 2016, Alcoa Corporation had an outstanding receivable of $13 and $11, respectively, from the smelting, rolling mill, and mining and refining companies for labor and other employee-related expenses. Capital investment in the project is expected to total approximately $10,800 (SAR 40.5 billion) and has been funded through a combination of equity contributions by the joint venture partners and project financing obtained by the joint venture companies, which has been partially guaranteed by both partners (see below). Both the equity contributions and the guarantees of the project financing are based on the joint venture’s partners’ ownership interests. Originally, it was estimated that Alcoa Corporation’s total equity contribution in the joint venture related to the capital investment in the project would be approximately $1,100, of which Alcoa Corporation has contributed $982, including $1 in 2016. Based on changes to both the project’s capital investment and equity and debt structure from the initial plans, the estimated $1,100 equity contribution may be reduced. Separate from the capital investment in the project, Alcoa Corporation contributed $66 (Ma’aden contributed $199) to the joint venture in 2017 for short-term funding purposes in accordance with the terms of the joint venture companies’ financing arrangements. Both partners may be required to make such additional contributions in future periods. As of December 31, 2017 and 2016, the carrying value of Alcoa Corporation’s investment in this joint venture was $887 and $853, respectively. The rolling mill and mining and refining companies have project financing totaling $3,334 (reflects principal repayments made through December 31, 2017), of which a combined $837 represents Alcoa Corporation’s and AWAC’s respective 25.1% interest in the rolling mill company and the mining and refining company. Alcoa Corporation, itself and on behalf of AWAC, has issued guarantees (see below) to the lenders in the event of default on the debt service requirements by the rolling mill company through 2018 and 2021 and by the mining and refining company through 2019 and 2024 (Ma’aden issued similar guarantees related to its 74.9% interest). Alcoa Corporation’s guarantees for the rolling mill and mining and refining companies cover total debt service requirements of $132 in principal and up to a maximum of approximately $25 in interest per year (based on projected interest rates). Previously, Alcoa Corporation issued similar guarantees related to the project financing of the smelting company. In December 2017, the smelting company refinanced and/or amended all of its existing outstanding debt. The guarantees that were previously required of the Company were effectively terminated. At December 31, 2017 and 2016, the combined fair value of the guarantees was $3 and $6, respectively, which was included in Other noncurrent liabilities and deferred credits on the accompanying Consolidated Balance Sheet. In the event Alcoa Corporation would be required to make payments under the guarantees related to the mining and refining company, 40% of such amount would be contributed to Alcoa Corporation by Alumina Limited, consistent with its ownership interest in AWAC. As a result of the Separation Transaction, the various lenders to the joint venture companies required Arconic to maintain joint and several guarantees with Alcoa Corporation. In the event of default by any of the joint venture companies, the lenders would make a claim against both Alcoa Corporation and Arconic. Accordingly, Alcoa Corporation would perform under its guarantee; however, if the Company failed to perform, Arconic would be required to perform under its own guarantee. Arconic would then subsequently seek indemnification from Alcoa Corporation under the terms of the Separation and Distribution Agreement. DBNGP Trust mid-2016, In April 2016, AofA sold its 20% interest in the consortium, effectively terminating its remaining obligation to make contributions under the most recent equity call plan, to the only other member of the consortium, DUET Group. AofA received $145 (A$192) in cash, which was included in Sales of investments on the accompanying Statement of Consolidated Cash Flows, and recorded a gain of $27 (A$35) ($11 (A$15) after-tax

Inventories

Inventories	12 Months Ended
	Dec. 31, 2017
Inventory Disclosure [Abstract]
Inventories	I. Inventories December 31, 2017 2016 Finished goods $ 296 $ 226 Work-in-process 258 220 Bauxite and alumina 585 429 Purchased raw materials 473 363 Operating supplies 147 137 LIFO reserve (306 ) (215 ) $ 1,453 $ 1,160 At December 31, 2017 and 2016, the total amount of inventories valued on a LIFO basis was $516, or 29%, and $393, or 29%, respectively, of total inventories before LIFO adjustments. The inventory values, prior to the application of LIFO, are generally determined under the average cost method, which approximates current cost.

Properties, Plants, and Equipme

Properties, Plants, and Equipment, Net	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Properties, Plants, and Equipment, Net	J. Properties, Plants, and Equipment, Net December 31, 2017 2016 Land and land rights, including mines $ 346 $ 346 Structures (by type of operation): Bauxite mining 1,250 1,194 Alumina refining 2,664 2,500 Aluminum smelting and casting 3,575 3,544 Energy generation 552 610 Aluminum rolling 298 287 Other 417 405 8,756 8,540 Machinery and equipment (by type of operation): Bauxite mining 508 465 Alumina refining 4,009 3,773 Aluminum smelting and casting 6,827 6,655 Energy generation 905 1,080 Aluminum rolling 1,020 884 Other 288 309 13,557 13,166 22,659 22,052 Less: accumulated depreciation, depletion, and amortization 13,908 13,225 8,751 8,827 Construction work-in-progress 387 498 $ 9,138 $ 9,325 As of December 31, 2017 and 2016, the net carrying value of temporarily idled refining assets was $141 and $158, respectively, representing 2,305 kmt of idle capacity in both periods. Also, as of December 31, 2017 and 2016, the net carrying value of temporarily idled smelting assets was $248 and $314, respectively, representing 856 kmt and 778 kmt, respectively, of idle capacity. In July 2017, Alcoa Corporation, recategorized 269 kmt of smelting capacity to idle capacity due to the decision to partially restart (161 kmt) a previously permanently closed smelter (carrying value is zero —see 2017 Actions in Note D), which is expected to be completed in the second quarter of 2018. Additionally, in December 2017, Alcoa Corporation permanently closed 191 kmt of smelting capacity, which was included in the 778 kmt of idle capacity as of December 31, 2016.

Goodwill and Other Intangible A

Goodwill and Other Intangible Assets	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Goodwill and Other Intangible Assets	K. Goodwill and Other Intangible Assets Goodwill, which is included in Other noncurrent assets on the accompanying Consolidated Balance Sheet, was as follows: December 31, 2017 2016 Bauxite $ 2 $ 2 Alumina 4 4 Aluminum (1) - - Corporate (2) 148 149 $ 154 $ 155 (1) The carrying value of Aluminum’s goodwill is zero, comprised of goodwill of $989 and accumulated impairment losses of $989 as of both December 31, 2017 and 2016. Additionally, the carrying value of Corporate’s goodwill is net of accumulated impairment losses of $742 as of both December 31, 2017 and 2016. (2) As of December 31, 2017, the $148 of goodwill reflected in Corporate is allocated to two of Alcoa Corporation’s three reportable segments ($49 to Bauxite and $99 to Alumina) for purposes of impairment testing (see Note B). This goodwill is reflected in Corporate for segment reporting purposes because it is not included in management’s assessment of performance by the two reportable segments. Other intangible assets, which are included in Other noncurrent assets on the accompanying Consolidated Balance Sheet, were as follows: 2017 2016 December 31, Gross carrying amount Accumulated amortization Gross carrying amount Accumulated Computer software $ 241 $ (212 ) $ 252 $ (196 ) Patents and licenses* 25 (6 ) 70 (6 ) Other intangibles 21 (7 ) 21 (6 ) Total other intangible assets $ 287 $ (225 ) $ 343 $ (208 ) * As of December 31, 2016, Patents and licenses include amounts related to the capitalization of costs associated with the renewal of Alcoa Corporation’s FERC (Federal Energy Regulatory Commission) license at its Yadkin Hydroelectric Project, which was divested in February 2017 (see Note C). Computer software consists primarily of software costs associated with an enterprise business solution within Alcoa Corporation to drive common systems among all businesses. Amortization expense related to the intangible assets in the tables above for the years ended December 31, 2017, 2016, and 2015 was $12, $7, and $10, respectively, and is expected to be approximately $10 annually from 2018 to 2022.

Debt

Debt	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Debt	L. Debt Long-Term Debt. December 31, 2017 2016 6.75% Notes, due 2024 $ 750 $ 750 7.00% Notes, due 2026 500 500 BNDES Loans, due 2018-2029 (see below for weighted average rates) 137 192 Other 50 38 Unamortized discounts and deferred financing costs (33 ) (35 ) 1,404 1,445 Less: amount due within one year 16 21 $ 1,388 $ 1,424 The principal amount of long-term debt maturing in each of the next five years is $16 in 2018, $30 in 2019, and $15 in each of 2020, 2021, and 2022. 144A Debt ANBHV has the option to redeem the Notes on at least 30 days, but not more than 60 days, prior notice to the holders of the Notes under multiple scenarios, including, in whole or in part, at any time or from time to time after September 2019, in the case of the 2024 Notes, or after September 2021, in the case of the 2026 Notes, at a redemption price specified in the indenture (up to 105.063% of the principal amount for the 2024 Notes and up to 103.500% of the principal amount of the 2026 Notes, plus any accrued and unpaid interest in each case). Also, the Notes are subject to repurchase upon the occurrence of a change in control repurchase event (as defined in the indenture) at a repurchase price in cash equal to 101% of the aggregate principal amount of the Notes repurchased, plus any accrued and unpaid interest on the Notes repurchased. The Notes are senior unsecured obligations of ANHBV and do not entitle the holders to any registration rights pursuant to a registration rights agreement. ANHBV does not intend to file a registration statement with respect to resales of or an exchange offer for the Notes. The Notes are guaranteed on a senior unsecured basis by Alcoa Corporation and its subsidiaries that are guarantors under the Revolving Credit Agreement described below (the “subsidiary guarantors” and, together with Alcoa Corporation, the “guarantors”). Each of the subsidiary guarantors will be released from their Notes guarantees upon the occurrence of certain events, including the release of such guarantor from its obligations as a guarantor under the Revolving Credit Agreement. The Notes indenture contains various restrictive covenants similar to those described below for the Amended Revolving Credit Agreement, including a limitation on restricted payments, with, among other exceptions, capacity to pay annual ordinary dividends. Under the indenture, Alcoa Corporation may declare and make annual ordinary dividends in an aggregate amount not to exceed $38 in each of the November 1, 2016 through December 31, 2017 time period (no such dividends were made) and annual 2018, $50 in each of annual 2019 and 2020, and $75 in the January 1, 2021 through September 30, 2026 (maturity date of the 2026 Notes) time period, except that 50% of any unused amount of the base amount in any of the specified time periods may be used in the next succeeding period following the use of the base amount in said time period. Additionally, the restricted payments negative covenant includes a general exception to allow for potential future transactions incremental to those specifically provided for in the Notes indenture. This general exception provides for an aggregate amount of restricted payment not to exceed the greater of $250 and 1.5% of Alcoa Corporation’s consolidated total assets. Accordingly, Alcoa Corporation may make annual ordinary dividends in any fiscal year by an aggregate amount of up to $250, assuming no other restricted payments have reduced, in part or whole, the available limit. The limits of the restricted payments negative covenant under the Amended Revolving Credit Agreement (see Credit Facility below) would govern the amount of ordinary dividend payments the Company could make in a given timeframe if the allowed amount is less than the limits of the restricted payments negative covenant under the Notes indenture. In conjunction with this debt offering, the net proceeds of $1,228, plus an additional $81 of ParentCo cash on hand, were required to be placed in escrow contingent on completion of the Separation Transaction. The $81 represented the necessary cash to fund the redemption of the Notes, pay all regularly scheduled interest on the Notes through a specified date as defined in the indenture, and a premium on the principal of the Notes if the Separation Transaction had not been completed by a certain time as defined in the indenture. As a result, the $1,228 of escrowed cash was recorded as restricted cash. The issuance of the Notes and the increase in restricted cash both in the amount of $1,228 were not reflected in the accompanying Statement of Consolidated Cash Flows as these represent noncash financing and investing activities, respectively. The subsequent release of the $1,228 from escrow occurred on October 31, 2016 in preparation for the Separation Transaction. This decrease in restricted cash was reflected in the accompanying Statement of Consolidated Cash Flows as a cash inflow in the Net change in restricted cash line item. BNDES Loans Additionally, Alumínio had a loan agreement with BNDES that provided for a financing commitment of $85 (R$177), which also was used to pay for certain expenditures of the Estreito hydroelectric power project. Interest on the loan was a Brazil real rate of interest equal to BNDES’ long-term interest rate plus a margin of 1.55%. Principal and interest were payable monthly, which began in January 2013 and were originally scheduled to end in September 2029. During 2017 and 2016, Alumínio repaid $44 (R$138) and $3 (R$11), respectively, of outstanding borrowings. The repayments in 2017 include an early repayment of $41 (R$131) made in August, representing the remaining outstanding loan balance. This early repayment was made without penalty under the approval of BNDES. With the full repayment of this loan, the commitment was effectively terminated. As of December 31, 2016, Alumínio’s outstanding borrowings were $42 (R$137) and the interest rate was 9.05%. Credit Facility. The Amended Revolving Credit Agreement provides a $1,500 senior secured revolving credit facility (the “Revolving Credit Facility”) to be used for working capital and/or other general corporate purposes of Alcoa Corporation and its subsidiaries. Subject to the terms and conditions of the Amended Revolving Credit Agreement, ANHBV may from time to time request the issuance of letters of credit up to $750 under the Revolving Credit Facility, subject to a sublimit of $400 for any letters of credit issued for the account of Alcoa Corporation or any of its domestic subsidiaries. Additionally, ANHBV may from time to time request that each of the lenders provide one or more additional tranches of term loans and/or increase the aggregate amount of revolving commitments, together in an aggregate principal amount of up to $500 (previously $0). The Revolving Credit Facility is scheduled to mature on November 14, 2022 (previously November 1, 2021), unless extended or earlier terminated in accordance with the provisions of the Amended Revolving Credit Agreement. ANHBV may make extension requests during the term of the Revolving Credit Facility, subject to the lender consent requirements set forth in the Amended Revolving Credit Agreement. Under the provisions of the Amended Revolving Credit Agreement, ANHBV will pay a quarterly commitment fee ranging from 0.225% to 0.450% (based on Alcoa Corporation’s leverage ratio) on the unused portion of the Revolving Credit Facility. A maximum of $750 in outstanding borrowings under the Revolving Credit Facility may be denominated in euros. Loans will bear interest at a rate per annum equal to an applicable margin plus, at ANHBV’s option, either (a) an adjusted LIBOR rate or (b) a base rate determined by reference to the highest of (1) the prime rate of JPMorgan Chase Bank, N.A., (2) the greater of the federal funds effective rate and the overnight bank funding rate, plus 0.5%, and (3) the one month adjusted LIBOR rate plus 1% per annum. The applicable margin for all loans will vary based on Alcoa Corporation’s leverage ratio and will range from 1.75% to 2.50% for LIBOR loans and 0.75% to 1.50% for base rate loans. Outstanding borrowings may be prepaid without premium or penalty, subject to customary breakage costs. All obligations of Alcoa Corporation or a domestic entity under the Revolving Credit Facility are secured by, subject to certain exceptions (including a limitation of pledges of equity interests in certain foreign subsidiaries to 65%, and certain thresholds with respect to real property), a first priority lien on substantially all assets of Alcoa Corporation and the material domestic wholly-owned subsidiaries of Alcoa Corporation and certain equity interests of specified non-U.S. The Amended Revolving Credit Agreement includes a number of customary affirmative covenants. Additionally, the Amended Revolving Credit Agreement contains a number of negative covenants (applicable to Alcoa Corporation and certain subsidiaries described as restricted), that, subject to certain exceptions, include limitations on (among other things): liens; fundamental changes; sales of assets; indebtedness (see below); entering into restrictive agreements; restricted payments (see below), including repurchases of common stock and shareholder dividends (see below); investments (see below), loans, advances, guarantees, and acquisitions; transactions with affiliates; amendment of certain material documents; and a covenant prohibiting reductions in the ownership of AWAC entities, and certain other specified restricted subsidiaries of Alcoa Corporation, below an agreed level. The Amended Revolving Credit Agreement also includes financial covenants requiring the maintenance of a specified interest expense coverage ratio of not less than 5.00 to 1.00, and a leverage ratio for any period of four consecutive fiscal quarters that is not greater than 2.25 to 1.00 (may be increased to a level not higher than 2.50 to 1.00). As of December 31, 2017 and 2016, Alcoa Corporation was in compliance with all such covenants. The indebtedness, restricted payments, and investments negative covenants include general exceptions to allow for potential future transactions incremental to those specifically provided for in the Amended Revolving Credit Agreement. The indebtedness negative covenant provides for an incremental amount not to exceed the greater of $1,000 (previously $500) and 6.0% (previously 3.0%) of Alcoa Corporation’s consolidated total assets. Additionally, the restricted payments negative covenant provides for an aggregate amount not to exceed $100 (previously $0) and the investments negative covenant provides for an aggregate amount not to exceed $400 (previously $250), both of which contain two conditions in which these limits may increase. First, in any fiscal year, the thresholds for the restricted payments and investments negative covenants increase by $250 (previously $0) and $200 (previously $0), respectively, if the consolidated net leverage ratio is not greater than 1.20 to 1.00 and 1.30 to 1.00, respectively, as of the end of the prior fiscal year. Secondly, in regards to both the $100 and $250 for restricted payments and the $200 for investments, 50% of any unused amount of these base amounts in any fiscal year may be used in the next succeeding fiscal year. The following describes the specific restricted payment negative covenant for share repurchases and the application of the restricted payments general exception (described above) to both share repurchases and ordinary dividend payments. Alcoa Corporation may repurchase shares of its common stock pursuant to stock option exercises and benefit plans in an aggregate amount not to exceed $25 during any fiscal year, except that 50% of any unused amount of the base amount in any fiscal year may be used in the next succeeding fiscal year following the use of the base amount in said fiscal year. Additionally, as described above, the Amended Revolving Credit Agreement provides general exceptions to the restricted payments negative covenant that would allow Alcoa Corporation to exceed this specified threshold for share repurchases in any fiscal year by an aggregate amount of up to $100 (see above for conditions that provide for this limit to increase), assuming no other restricted payments have reduced, in part or whole, the available limit. Also, the Amended Revolving Credit Agreement rescinded the specific terms included in the Revolving Credit Agreement related to ordinary dividend payments. Previously, Alcoa Corporation was able to declare and make annual ordinary dividends in an aggregate amount not to exceed $38 in each of the November 1, 2016 through December 31, 2017 time period (no such dividends were made) and annual 2018, $50 in each of annual 2019 and 2020, and $75 in the January 1, 2021 through November 1, 2021 time period, except that 50% of any unused amount of the base amount in any of the specified time periods may be used in the next succeeding period following the use of the base amount in said time period. Under the Amended Revolving Credit Agreement, any ordinary dividend payments made by Alcoa Corporation are only subject to the general exception for restricted payments described above. Accordingly, Alcoa Corporation may make annual ordinary dividends in any fiscal year by an aggregate amount of up to $100 (see above for conditions that provide for this limit to increase), assuming no other restricted payments have reduced, in part or whole, the available limit. The limits of the restricted payments negative covenant under the Notes indenture (see 144A Debt above) would govern the amount of ordinary dividend payments the Company could make in a given timeframe if the allowed amount is less than the limits of the restricted payments negative covenant under the Amended Revolving Credit Agreement. The Amended Revolving Credit Agreement contains customary events of default, including with respect to a failure to make payments under the Revolving Credit Facility, cross-default and cross-judgment default, and certain bankruptcy and insolvency events. There were no amounts outstanding at December 31, 2017 and 2016 and no amounts were borrowed during 2017 and 2016 (September 16 th through December 31 st ) under the Revolving Credit Facility.

Preferred and Common Stock

Preferred and Common Stock	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Preferred and Common Stock	M. Preferred and Common Stock Preferred Stock. Common Stock. As of December 31, 2017, 27,270,482 shares of common stock were available for issuance under Alcoa Corporation’s employee stock-based compensation plan. Alcoa Corporation issues new shares to satisfy the exercise of stock options and the conversion of stock awards. Stock-based Compensation For all periods prior to the Separation Date, Alcoa Corporation’s employees participated in ParentCo’s stock-based compensation plan. The stock-based compensation expense recorded by Alcoa Corporation in the referenced periods includes expense associated with employees historically attributable to Alcoa Corporation’s operations and an allocation of expense (see Note A) related to ParentCo’s corporate employees. For 2017 and the last two months of 2016, Alcoa Corporation employees participated in the Company’s stock-based compensation plan. Effective November 1, 2016, all outstanding stock options (vested and non-vested) non-vested The following description of Alcoa Corporation’s stock-based compensation plan is not materially different from the description of ParentCo’s stock-based compensation plan prior to the Separation Transaction. Alcoa Corporation has a stock-based compensation plan under which stock options and stock awards generally will be granted in either January or February each year to eligible employees (the Company’s Board of Directors also receive certain stock awards; however, these amounts are not material). Most plan participants can choose whether to receive their award in the form of stock options, stock awards, or a combination of both. This choice is made before the grant is issued and is irrevocable. Stock options are granted at the closing market price of Alcoa Corporation’s common stock on the date of grant and vest over a three-year service period (1/3 each year) with a ten-year The final number of market-based and performance-based stock awards earned is based on Alcoa Corporation’s achievement of certain targets over a three-year measurement period. For market-based stock awards granted in 2017, the award will be earned at the end of the vesting period based on the Company’s total shareholder return measured against the total shareholder return of the Standard & Poor’s 500® Index from January 1, 2017 through December 31, 2019. For performance-based stock awards granted in 2017, the award will be earned at the end of the vesting period based on the Company’s performance against a pre-established return-on-capital target measured from January 1, 2017 through December 31, 2019. For performance-based stock awards granted in 2016 and 2015, one-third of the award was to be earned each year during the vesting period based on the Company’s performance against a pre-established return-on-capital target for that year measured from January 1st through December 31st (the second tranche of the 2016 performance awards and the third tranche of the 2015 performance awards were not earned in 2017). All market-based and performance-based stock awards earned over the three-year service period vest on the third anniversary of the award grant date. In 2017, 2016, and 2015, Alcoa Corporation recognized stock-based compensation expense of $24, $28, and $35, respectively, of which between 80% and 90% related to stock awards in each period (there was no stock-based compensation expense capitalized in 2017, 2016, or 2015). Of the total pretax stock-based compensation expense recognized in 2016 and 2015, $16 and $21, respectively, relates to the allocation of expense for ParentCo’s corporate employees. As part of both Alcoa Corporation’s and ParentCo’s stock-based compensation plan design in the respective periods, individuals who are retirement-eligible have a six-month Stock-based compensation expense is based on the grant date fair value of the applicable equity grant. For stock awards with no performance or market condition and for stock awards with a performance condition, the fair value was equivalent to the closing market price of either Alcoa Corporation’s or ParentCo’s common stock on the date of grant in the respective periods. For stock awards with a market condition, the fair value was estimated on the date of grant using a Monte Carlo simulation model, which generated a result of $52.01 per award in 2017. The Monte Carlo simulation model uses certain assumptions to estimate the fair value of a market-based stock award, including volatility (36.98% for Alcoa Corporation and 11.44% for the Standard & Poor’s 500 ® The following describes in detail the assumptions used by Alcoa Corporation to estimate the fair value of stock options granted in 2017 (the assumptions used to estimate the fair value of stock options granted by ParentCo in 2016 and 2015 were not materially different). As the Company was a standalone publicly-traded company only for a two-month pre- For stock options outstanding as of October 31, 2016 that were originally granted under ParentCo’s stock-based compensation plan to Alcoa Corporation employees, the previously-mentioned fair values were adjusted to reflect both the impact of ParentCo’s 1-for-3 The activity for stock options and stock awards during 2017 was as follows: Stock options Stock awards Number of options Weighted average exercise price Number of awards Weighted average FMV per award Outstanding, January 1, 2017 4,108,535 $ 24.69 2,557,842 $ 22.65 Granted 331,681 37.68 796,330 37.39 Exercised (1,679,148 ) 25.80 - - Converted - - (764,064 ) 24.98 Expired or forfeited (73,961 ) 28.99 (145,756 ) 24.90 Performance share adjustment - - (143,020 ) 21.03 Outstanding, December 31, 2017 2,687,107 25.48 2,301,332 26.93 As of December 31, 2017, the number of stock options outstanding had a weighted average remaining contractual life of 5.82 years and a total intrinsic value of $76. Additionally, 1,704,299 of the stock options outstanding were fully vested and exercisable and had a weighted average remaining contractual life of 4.47 years, a weighted average exercise price of $26.19, and a total intrinsic value of $47 as of December 31, 2017. Cash received from stock option exercises was $43 and $10 in 2017 and 2016, respectively, and the total intrinsic value of stock options exercised during 2017 and 2016 was $24 and $4, respectively. At December 31, 2017, there was $24 (pretax) of unrecognized compensation expense related to non-vested non-vested

Pension and Other Postretiremen

Pension and Other Postretirement Benefits	12 Months Ended
	Dec. 31, 2017
Retirement Benefits [Abstract]
Pension and Other Postretirement Benefits	N. Pension and Other Postretirement Benefits Alcoa Corporation maintains pension plans covering most U.S. employees and certain employees in foreign locations (see Note T). Pension benefits generally depend on length of service, job grade, and remuneration. Substantially all benefits are paid through pension trusts that are sufficiently funded to ensure that all plans can pay benefits to retirees as they become due. Most salaried and non-bargaining The Company also maintains health care and life insurance postretirement benefit plans covering eligible U.S. retired employees and certain retirees from foreign locations (see Note T). Generally, the medical plans are unfunded and pay a percentage of medical expenses, reduced by deductibles and other coverage. Life benefits are generally provided by insurance contracts. Alcoa Corporation retains the right, subject to existing agreements, to change or eliminate these benefits. All salaried and certain non-bargaining The above descriptions of retirement benefits for Alcoa Corporation participants also describe the retirement benefits provided by ParentCo to its employees and retirees prior to the Separation Date. For all periods prior to August 1, 2016 (see below), eligible employees attributable to Alcoa Corporation operations participated in the U.S. defined benefit pension and other postretirement benefit plans sponsored by ParentCo (the “Shared Plans”), which included Arconic and ParentCo corporate participants. Alcoa Corporation accounted for its portion of the Shared Plans as multiemployer benefit plans. Accordingly, Alcoa Corporation did not record an asset or liability to recognize the funded status of the Shared Plans. The multiemployer contribution expense attributable to employees of Alcoa Corporation-related operations was based primarily on pensionable compensation of such employees for the pension plans and estimated interest costs for the other postretirement benefit plans. Additionally, for all periods prior to August 1, 2016, Alcoa Corporation recorded an allocation of expenses for the Shared Plans attributable to ParentCo corporate participants, as well as to closed and sold operations (see Cost Allocations in Note A). Also, certain of the ParentCo plans described above were specific to employees attributable to Alcoa Corporation operations (non-U.S.) In preparation for the Separation Transaction, effective August 1, 2016, certain of the Shared Plans were separated into standalone plans for both Alcoa Corporation (the “New Direct Plans”) and Arconic. Accordingly, the New Direct Plans for Alcoa Corporation were measured as of August 1, 2016. One of the primary assumptions used to measure the New Direct Plans was a weighted average discount rate of 3.48%. This measurement yielded a combined net unfunded status of $2,348. Additionally, certain other Shared Plans were assumed by Alcoa Corporation (the “Additional New Direct Plans,” and collectively with the Direct Plans and New Direct Plans, the “Cumulative Direct Plans”) that did not require to be separated and/or to be remeasured. The Additional New Direct Plans had a combined net unfunded status of $180. The aggregate combined net unfunded status of the New Direct Plans and the Additional New Direct Plans was recognized in Alcoa Corporation’s Consolidated Balance Sheet at that time, consisting of a current liability of $136 and a noncurrent liability of $2,392. Additionally, Alcoa Corporation recognized $2,704 in Accumulated other comprehensive loss. The following table summarizes the total expenses recognized by Alcoa Corporation related to all pension and other postretirement benefits: Pension benefits Other postretirement benefits Type of Plan Type of Expense 2017 2016 2015 2017 2016 2015 Cumulative Direct Plans Net periodic benefit cost $ 119 $ 83 $ 106 $ 50 $ 21 $ (12 ) Shared Plans Multiemployer contribution expense - 28 64 - 12 32 Shared Plans Cost allocation - 25 84 - 8 11 $ 119 $ 136 $ 254 $ 50 $ 41 $ 31 The funded status of Alcoa Corporation’s Cumulative Direct Plans is measured as of December 31 each calendar year. All of the information that follows for pension and other postretirement benefit plans is only applicable to the Cumulative Direct Plans, as appropriate. Obligations and Funded Status Pension benefits Other December 31, 2017 2016 2017 2016 Change in benefit obligation Benefit obligation at beginning of year $ 7,269 $ 2,246 $ 1,286 $ 82 Benefit obligation assumed on August 1, 2016 - 5,316 - 1,277 Service cost 84 74 5 2 Interest cost 250 142 38 16 Amendments 2 1 - - Actuarial losses (gains) 388 (244 ) (1 ) (33 ) Settlements (64 ) (80 ) - - Benefits paid, net of participants’ contributions (437 ) (218 ) (116 ) (61 ) Medicare Part D subsidy receipts - - 5 3 Foreign currency translation impact 147 32 1 - Benefit obligation at end of year* $ 7,639 $ 7,269 $ 1,218 $ 1,286 Change in plan assets Fair value of plan assets at beginning of year $ 5,421 $ 1,891 $ - $ - Fair value of plan assets assumed on August 1, 2016 - 4,065 - - Actual return on plan assets 187 (332 ) - - Employer contributions 111 72 - - Participant contributions 15 18 - - Benefits paid (432 ) (224 ) - - Administrative expenses (41 ) (11 ) - - Settlements (62 ) (80 ) - - Foreign currency translation impact 123 22 - - Fair value of plan assets at end of year* $ 5,322 $ 5,421 $ - $ - Funded status* $ (2,317 ) $ (1,848 ) $ (1,218 ) $ (1,286 ) Less: Amounts attributed to joint venture partners (37 ) (30 ) - - Net funded status $ (2,280 ) $ (1,818 ) $ (1,218 ) $ (1,286 ) Amounts recognized in the Consolidated Balance Sheet consist of: Noncurrent assets $ 72 $ 43 $ - $ - Current liabilities (11 ) (10 ) (118 ) (120 ) Noncurrent liabilities (2,341 ) (1,851 ) (1,100 ) (1,166 ) Net amount recognized $ (2,280 ) $ (1,818 ) $ (1,218 ) $ (1,286 ) Amounts recognized in Accumulated Other Comprehensive Loss consist of: Net actuarial loss $ 3,743 $ 3,254 $ 281 $ 295 Prior service cost (benefit) 35 42 (30 ) (36 ) Total, before tax effect 3,778 3,296 251 259 Less: Amounts attributed to joint venture partners 45 36 - - Net amount recognized, before tax effect $ 3,733 $ 3,260 $ 251 $ 259 Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Loss consist of: Net actuarial loss (benefit) $ 676 $ 337 $ (1 ) $ (33 ) Amortization of accumulated net actuarial loss (187 ) (105 ) (13 ) (8 ) Prior service cost (benefit) 2 2 - (1 ) Amortization of prior service (cost) benefit (9 ) (7 ) 6 5 Total, before tax effect 482 227 (8 ) (37 ) Less: Amounts attributed to joint venture partners 9 (3 ) - - Net amount recognized, before tax effect $ 473 $ 230 $ (8 ) $ (37 ) * At December 31, 2017, the benefit obligation, fair value of plan assets, and funded status for U.S. pension plans were $5,093, $3,195, and $(1,898), respectively. At December 31, 2016, the benefit obligation, fair value of plan assets, and funded status for U.S. pension plans were $4,977, $3,504, and $(1,473), respectively. Pension Plan Benefit Obligations Pension benefits 2017 2016 The aggregate projected benefit obligation and accumulated benefit obligation for all defined benefit pension plans was as follows: Projected benefit obligation $ 7,639 $ 7,269 Accumulated benefit obligation 7,426 7,075 The aggregate projected benefit obligation and fair value of plan assets for pension plans with projected benefit obligations in excess of plan assets was as follows: Projected benefit obligation 7,061 6,699 Fair value of plan assets 4,671 4,807 The aggregate accumulated benefit obligation and fair value of plan assets for pension plans with accumulated benefit obligations in excess of plan assets was as follows: Accumulated benefit obligation 6,885 6,531 Fair value of plan assets 4,671 4,807 Components of Net Periodic Benefit Cost Pension benefits (1) Other postretirement benefits (2) 2017 2016 2015 2017 2016 2015 Service cost $ 71 $ 61 $ 51 $ 5 $ 2 $ - Interest cost 244 138 89 38 16 4 Expected return on plan assets (398 ) (242 ) (121 ) - - - Recognized net actuarial loss 185 102 42 13 8 (3 ) Amortization of prior service cost (benefit) 9 7 6 (6 ) (5 ) (9 ) Settlements (3) 5 16 14 - - - Curtailments (4) - - 9 - - (4 ) Special termination benefits (5) 3 1 16 - - - Net periodic benefit cost (6) $ 119 $ 83 $ 106 $ 50 $ 21 $ (12 ) (1) In 2017 and 2016, net periodic benefit cost for U.S pension plans was $74 and $21, respectively. (2) In 2017 and 2016, net periodic benefit cost for other postretirement benefits reflects a reduction of $8 and $6, respectively, related to the recognition of the federal subsidy awarded under Medicare Part D. (3) In 2017 and 2016, settlements were due to workforce reductions (see Note D). In 2015, settlements were due to workforce reductions (see Note D) and the payment of lump sum benefits. (4) In 2015, curtailments were due to elimination of benefits or workforce reductions (see Note D). (5) In 2017, 2016, and 2015, special termination benefits were due to workforce reductions (see Note D). (6) Amounts attributed to joint venture partners are not included. Amounts Expected to be Recognized in Net Periodic Benefit Cost Pension benefits Other postretirement benefits 2018 2018 Net actuarial loss recognition $ 223 $ 15 Prior service cost (benefit) recognition 9 (6 ) Assumptions Weighted average assumptions used to determine benefit obligations for pension and other postretirement benefit plans were as follows: December 31, 2017 2016 Discount rate—pension plans 3.68 % 4.12 % Discount rate—other postretirement benefit plans 3.54 3.93 Rate of compensation increase—pension plans 3.28 3.61 The discount rate is determined using a Company-specific yield curve model (above-median) developed with the assistance of an external actuary. The cash flows of the plans’ projected benefit obligations are discounted using a single equivalent rate derived from yields on high quality corporate bonds, which represent a broad diversification of issuers in various sectors. The yield curve model parallels the plans’ projected cash flows, which have a weighted average duration of 12 years, and the underlying cash flows of the bonds included in the model exceed the cash flows needed to satisfy the Company’s plans’ obligations multiple times. If a deep market of high quality corporate bonds does not exist in a country, then the yield on government bonds plus a corporate bond yield spread is used. The rate of compensation increase is based upon anticipated compensation increases and estimated inflation. For 2018, the rate of compensation increase will be 3.28%. Weighted average assumptions used to determine net periodic benefit cost for pension and other postretirement benefit plans were as follows: 2017 2016 2015 Discount rate—pension plans* 3.61 % 3.45 % 4.09 % Discount rate—other postretirement benefit plans* 3.30 2.90 4.15 Expected long-term rate of return on plan assets—pension plans 7.47 7.31 6.91 Rate of compensation increase—pension plans 3.61 3.65 3.74 * In all periods presented, the respective discount rates were used to determine net periodic benefit cost for most plans for the full annual period. However, the discount rates for a limited number of plans were updated during 2017, 2016, and 2015 to reflect the remeasurement of these plans due to settlements and/or curtailments. The updated discount rates used were not significantly different from the discount rates presented. The expected long-term rate of return on plan assets is generally applied to a five-year market-related value of plan assets (a four-year average or the fair value at the plan measurement date is used for certain non-U.S. Assumed health care cost trend rates for U.S. other postretirement benefit plans were as follows (non-U.S. 2017 2016 2015 Health care cost trend rate assumed for next year 5.5 % 5.5 % 5.5 % Rate to which the cost trend rate gradually declines 4.5 % 4.5 % 4.5 % Year that the rate reaches the rate at which it is assumed to remain 2021 2020 2019 The assumed health care cost trend rate is used to measure the expected cost of gross eligible charges covered by Alcoa Corporation’s other postretirement benefit plans. For 2018, a 5.5% trend rate will be used, reflecting management’s best estimate of the change in future health care costs covered by the plans. The plans’ actual annual health care cost trend experience (based on ParentCo’s plans that previously included the Alcoa Corporation participants) over the past three years has ranged from (0.8)% to 9.0% Management does not believe this three-year range is indicative of expected increases for future health care costs over the long-term. Assumed health care cost trend rates have an effect on the amounts reported for a health care plan. A one-percentage 1% increase 1% Effect on other postretirement benefit obligations $ 79 $ (70 ) Effect on total of service and interest cost components 3 (2 ) Plan Assets Alcoa Corporation’s pension plan investment policy and weighted average asset allocations at December 31, 2017 and 2016, by asset class, were as follows: Plan assets Asset class Policy range 2017 2016 Equities 20–55 % 40 % 37 % Fixed income 25–55 % 35 36 Other investments 15–35 % 25 27 Total 100 % 100 % The principal objectives underlying the investment of the pension plans’ assets are to ensure that Alcoa Corporation can properly fund benefit obligations as they become due under a broad range of potential economic and financial scenarios, maximize the long-term investment return with an acceptable level of risk based on such obligations, and broadly diversify investments across and within various asset classes to protect asset values against adverse movements. Through 2017, specific objectives for long-term investment strategy included reducing the volatility of pension assets relative to pension liabilities and achieving risk factor diversification across the balance of the asset portfolio. A portion of the assets were matched to the interest rate profile of the benefit obligation through long duration fixed income investments and fixed income derivative instruments. Exposure to broad equity risk was decreased and diversified through investments in discretionary and systematic macro hedge funds, long/short equity hedge funds, and global and emerging market equities. Investments were further diversified by strategy, asset class, geography, and sector in an effort to enhance returns and mitigate downside risk. Several external investment managers were used to gain broad exposure to the financial markets and to mitigate manager-concentration risk. This investment strategy was defined prior to the Separation Date by ParentCo management and maintained by Alcoa Corporation management; however, this strategy resulted in investment returns less than those expected since the Separation Date. Accordingly, in 2018, management plans to implement a less-complex, peer-like investment strategy and, as a result, restructure the asset portfolio. This new strategy will result in investing a higher percentage of the portfolio in assets that will match the interest rate and credit risk profiles of the benefit obligations, as well as investing in assets with returns expected to exceed the actual returns of the previous asset mix. To achieve this new strategy, the portfolio will no longer include investments in discretionary and systematic macro hedge funds and long/short equity hedge funds. Instead, the portfolio will include a larger concentration of investments in long duration government debt, long-duration corporate credit, and real estate, as well as new investments in high yield and emerging sovereign debt and global-listed infrastructure. As a result, the expected asset class mix will be approximately 30% in equities, approximately 50% in fixed income, and approximately 20% in other investments. Additionally, the number of asset managers will be changed. The Company expects the new strategy to be fully implemented by the end of 2018. Investment practices comply with the requirements of applicable laws and regulations in the respective jurisdictions, including the Employee Retirement Income Security Act of 1974 (ERISA) in the United States. The use of derivative instruments by external investment managers is permitted where appropriate and necessary for achieving overall investment policy objectives and for mitigating interest rate and other asset class risks. The following section describes the valuation methodologies used by the trustees to measure the fair value of pension plan assets, including, if applicable, an indication of the level in the fair value hierarchy in which each type of asset is generally classified (see Note O for the definition of fair value and a description of the fair value hierarchy). Equities. non-U.S. Fixed income. non-U.S. Other investments. The fair value methods described above may not be indicative of net realizable value or reflective of future fair values. Additionally, while Alcoa Corporation believes the valuation methods used by the plans’ trustees are appropriate and consistent with other market participants, the use of different methodologies or assumptions to determine the fair value of certain financial instruments could result in a different fair value measurement at the reporting date. The following table presents the fair value of pension plan assets classified under either the appropriate level of the fair value hierarchy or net asset value: December 31, 2017 Level 1 Level 2 Level 3 Net Asset Total Equities: Equity securities $ 906 $ - $ - $ 869 $ 1,775 Long/short equity hedge funds - - - 152 152 Private equity - - - 226 226 $ 906 $ - $ - $ 1,247 $ 2,153 Fixed income: Intermediate and long duration government/credit $ 95 $ 378 $ - $ 264 $ 737 Cash and cash equivalent funds 313 - - 742 1,055 Other - 56 - - 56 $ 408 $ 434 $ - $ 1,006 $ 1,848 Other investments: Real estate $ 241 $ - $ - $ 365 $ 606 Discretionary and systematic macro hedge funds - - - 581 581 Other - - - 132 132 $ 241 $ - $ - $ 1,078 $ 1,319 Total (1) $ 1,555 $ 434 $ - $ 3,331 $ 5,320 December 31, 2016 Level 1 Level 2 Level 3 Net Asset Total Equities: Equity securities $ 520 $ - $ - $ 772 $ 1,292 Long/short equity hedge funds - - - 449 449 Private equity - - - 246 246 $ 520 $ - $ - $ 1,467 $ 1,987 Fixed income: Intermediate and long duration government/credit $ 78 $ 283 $ - $ 167 $ 528 Cash and cash equivalent funds 897 - - 468 1,365 Other - 61 - - 61 $ 975 $ 344 $ - $ 635 $ 1,954 Other investments: Real estate $ 88 $ - $ - $ 331 $ 419 Discretionary and systematic macro hedge funds - - - 866 866 Other 71 - - 139 210 $ 159 $ - $ - $ 1,336 $ 1,495 Total (2) $ 1,654 $ 344 $ - $ 3,438 $ 5,436 (1) As of December 31, 2017, the total fair value of pension plan assets excludes a net receivable of $2, which represents securities not yet settled plus interest and dividends earned on various investments, less an amount due to Arconic pension plans from Alcoa Corporation pension plans related to the separation of certain plans between the two companies. (2) As of December 31, 2016, the total fair value of pension plan assets excludes a net payable of $15, which represents an amount due to Arconic pension plans from Alcoa Corporation pension plans related to the separation of certain plans between the two companies. Funding and Cash Flows It is Alcoa Corporation’s policy to fund amounts for defined benefit pension plans sufficient to meet the minimum requirements set forth in applicable country benefits laws and tax laws, including the Pension Protection Act of 2006; the Worker, Retiree, and Employer Recovery Act of 2008; the Moving Ahead for Progress in the 21 st Century Act of 2012; the Highway and Transportation Funding Act of 2015; and the Bipartisan Budget Act of 2016 for U.S. plans. From time to time, Alcoa Corporation contributes additional amounts as deemed appropriate. In 2017, 2016, and 2015, cash contributions to Alcoa Corporation’s defined benefit pension plans were $106, $66, and $69. The minimum required contribution to defined benefit pension plans in 2018 is estimated to be $290, of which $250 is for U.S. plans. Additionally, the Company expects to make discretionary contributions of approximately $300 combined to the U.S. and Canadian defined benefit pension plans in 2018 (see Note T). Benefit payments expected to be paid to pension and other postretirement benefit plan participants and expected Medicare Part D subsidy receipts are as follows: Year ended December 31, Pension Gross Other Medicare Part D Net Other 2018 $ 490 $ 125 $ 10 $ 115 2019 485 125 10 115 2020 490 120 5 115 2021 490 120 5 115 2022 490 115 5 110 2023 through 2027 2,400 425 25 400 $ 4,845 $ 1,030 $ 60 $ 970 Defined Contribution Plans Alcoa Corporation sponsors savings and investment plans in several countries, including Australia and the United States. Prior to the Separation Date, employees attributable to Alcoa Corporation operations participated in ParentCo-sponsored plans. In the United States, employees may contribute a portion of their compensation to the plans, and Alcoa Corporation (ParentCo prior to Separation Date) matches a specified percentage of these contributions in equivalent form of the investments elected by the employee. Also, the Company makes contributions to a retirement savings account based on a percentage of eligible compensation for certain U.S. employees hired after March 1, 2006 that are not able to participate in Alcoa Corporation’s defined benefit pension plans (see Note T). Alcoa Corporation’s expenses related to all defined contribution plans were $65 in 2017, $57 in 2016, and $59 in 2015.

Derivatives and Other Financial

Derivatives and Other Financial Instruments	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Derivatives and Other Financial Instruments	O. Derivatives and Other Financial Instruments Fair Value. • Level 1—Unadjusted quoted prices in active markets that are accessible at the measurement date for identical, unrestricted assets or liabilities. • Level 2—Inputs other than quoted prices included within Level 1 that are observable for the asset or liability, either directly or indirectly, including quoted prices for similar assets or liabilities in active markets; quoted prices for identical or similar assets or liabilities in markets that are not active; inputs other than quoted prices that are observable for the asset or liability (e.g., interest rates); and inputs that are derived principally from or corroborated by observable market data by correlation or other means. • Level 3—Inputs that are both significant to the fair value measurement and unobservable. Derivatives. Alcoa Corporation’s commodity and derivative activities are subject to the management, direction, and control of the Strategic Risk Management Committee (SRMC), which consists of at least three members, including the chief executive officer and the chief financial officer. The remaining member(s) are other officers and/or employees of the Company as the chief executive officer may designate from time to time. Currently, the only other member of the SRMC is Alcoa Corporation’s treasurer. The SRMC meets on a periodic basis to review derivative positions and strategy and reports to the Audit Committee of Alcoa Corporation’s Board of Directors on the scope of its activities. Alcoa Corporation’s aluminum, energy, and foreign exchange contracts are held for purposes other than trading. They are used primarily to mitigate uncertainty and volatility, and to cover underlying exposures. Alcoa Corporation is not involved in trading activities for energy, weather derivatives, or other nonexchange commodity trading activities. Several of Alcoa Corporation’s aluminum, energy, and foreign exchange contracts are classified as Level 1 or Level 2 under the fair value hierarchy. The total fair value of these derivative contracts recorded as assets and liabilities was $44 and $117, respectively, at December 31, 2017 and $5 and $2, respectively, at December 31, 2016. Certain of these contracts are designated as either fair value or cash flow hedging instruments. Combined, in 2017, 2016, and 2015, Alcoa Corporation recognized a loss of $22, a loss of less than $1, and a gain of $4, respectively, in Other (income) expenses, net on the accompanying Statement of Consolidated Operations related to these contracts. Additionally, for the contracts designated as cash flow hedges, Alcoa Corporation recognized an unrealized loss of $92, an unrealized gain of $2, and an unrealized loss of $13 in Other comprehensive loss in 2017, 2016, and 2015, respectively. In addition to the Level 1 and 2 derivative instruments described above, Alcoa Corporation has derivative instruments classified as Level 3 under the fair value hierarchy. These instruments are composed of (i) both embedded aluminum derivatives and an embedded credit derivative related to energy supply contracts and (ii) freestanding financial contracts related to energy purchases in the spot market, all of which are associated with nine smelters and three refineries. Certain of the embedded aluminum derivatives and financial contracts were designated as cash flow hedging instruments. All of these Level 3 derivative instruments are described below in detail and are enumerated as D1 through D11. The following section describes the valuation methodologies used by Alcoa Corporation to measure its Level 3 derivative instruments at fair value. Derivative instruments classified as Level 3 in the fair value hierarchy represent those in which management has used at least one significant unobservable input in the valuation model. Alcoa Corporation uses a discounted cash flow model to fair value all Level 3 derivative instruments. Where appropriate, the description below includes the key inputs to those models and any significant assumptions. These valuation models are reviewed and tested at least on an annual basis. Inputs in the valuation models for Level 3 derivative instruments are composed of the following: (i) quoted market prices (e.g., aluminum prices on the 10-year 10-year D1 through D5 10-year D6 LME-linked D7 LME-linked D8 mid-2016, D9 10-year 10-year 20-year D10 and D11 In January 2017, Alcoa Corporation and the counterparty entered into a new financial contract (D11) to hedge the anticipated power requirements at this smelter for the period from August 2017 through July 2021 and amended the existing financial contract to both reduce the hedged amount of anticipated power requirements and to move up the effective termination date to July 31, 2017. The new financial contract has been designated as a cash flow hedge of future purchases of electricity. Unrealized gains and losses on the new financial contract were recorded in Other comprehensive loss on the accompanying Consolidated Balance Sheet while realized gains and losses were recorded (began in August 2017) in Cost of goods sold as electricity purchases were made from the spot market. The following table presents quantitative information related to the significant unobservable inputs described above for Level 3 derivative contracts: Fair value at December 31, 2017 Unobservable input Range ($ in full amounts) Assets: Embedded aluminum derivative (D7) $ - Interrelationship of future aluminum and oil prices Aluminum: $2,258 per metric ton in January 2018 to $2,299 per metric ton in October 2018 Oil: $67 per barrel in January 2018 to $64 per barrel in October 2018 Financial contract (D11) 197 Interrelationship of forward energy price and the Consumer Price Index and price of electricity beyond forward curve Electricity: $83.69 per megawatt hour in 2018 to $53.60 per megawatt hour in 2021 Liabilities: Embedded aluminum derivative (D1) 403 Interrelationship of LME price to the amount of megawatt hours of energy needed to produce the forecasted metric tons of aluminum Aluminum: $2,258 per metric ton in 2018 to $2,651 per metric ton in 2027 Electricity: rate of 4 million megawatt hours per year Embedded aluminum derivatives (D3 through D5) 685 Price of aluminum beyond forward curve Aluminum: $2,679 per metric ton in 2028 to $2,759 per metric ton in 2029 (two contracts) and $3,055 per metric ton in 2036 (one contract) Midwest premium: $0.0950 per pound in 2018 to $0.1150 per pound in 2029 (two contracts) and 2036 (one contract) Embedded aluminum derivative (D8) 34 Interrelationship of LME price to the amount of megawatt hours of energy needed to produce the forecasted metric tons of aluminum Aluminum: $2,258 per metric ton in 2018 to $2,317 per metric ton in 2019 Midwest premium: $0.0950 per pound in 2018 to $0.1150 per pound in 2019 Electricity: rate of 2 million megawatt hours per year Embedded aluminum derivative (D2) 24 Interrelationship of LME price to overall energy price Aluminum: $2,110 per metric ton in 2018 to $2,342 per metric ton in 2019 Embedded credit derivative (D9) 27 Estimated difference in credit spread of each of Alcoa Corporation and counterparty, and negotiated multiplier 3.38% (credit spreads: Alcoa Corporation—2.48% and counterparty—1.51%; multiplier: 3.49% (1.87% 2 ) The fair values of Level 3 derivative instruments recorded as assets and liabilities in the accompanying Consolidated Balance Sheet were as follows: Asset Derivatives December 31, 2017 December 31, 2016 Derivatives designated as hedging instruments: Fair value of derivative contracts—current: Embedded aluminum derivatives $ - $ 29 Financial contract 96 - Fair value of derivative contracts—noncurrent: Embedded aluminum derivatives - 468 Financial contract 101 - Total derivatives designated as hedging instruments $ 197 $ 497 Derivatives not designated as hedging instruments: Fair value of derivative contracts—current: Financial contract $ - $ 17 Total derivatives not designated as hedging instruments $ - $ 17 Total Asset Derivatives $ 197 $ 514 Liability Derivatives Derivatives designated as hedging instruments: Fair value of derivative contracts—current: Embedded aluminum derivatives $ 120 $ 17 Fair value of derivative contracts—noncurrent: Embedded aluminum derivatives 992 187 Total derivatives designated as hedging instruments $ 1,112 $ 204 Derivatives not designated as hedging instruments: Fair value of derivative contracts—current: Embedded aluminum derivative $ 28 $ 10 Embedded credit derivative 4 5 Fair value of derivative contracts—noncurrent: Embedded aluminum derivative 6 18 Embedded credit derivative 23 30 Total derivatives not designated as hedging instruments $ 61 $ 63 Total Liability Derivatives $ 1,173 $ 267 The following table shows the net fair values of the Level 3 derivative instruments at December 31, 2017 and the effect on these amounts of a hypothetical change (increase or decrease of 10%) in the market prices or rates that existed as of December 31, 2017: Fair value asset/(liability) Index change of + / -10% Embedded aluminum derivatives $ (1,146 ) $ 489 Embedded credit derivative (27 ) 3 Financial contract 197 55 The following tables present a reconciliation of activity for Level 3 derivative contracts: Assets Liabilities 2017 Embedded aluminum derivatives Financial contracts Embedded aluminum derivatives Embedded credit derivative Opening balance—January 1, 2017 $ 497 $ 17 $ 232 $ 35 Total gains or losses (realized and unrealized) included in: Sales 3 - (110 ) - Cost of goods sold - (31 ) - (5 ) Other income, net 1 (7 ) 18 (3 ) Other comprehensive loss (499 ) 88 1,022 - Purchases, sales, issuances, and settlements* - 119 - - Transfers into and/or out of Level 3* - - - - Other (2 ) 11 (16 ) - Closing balance—December 31, 2017 $ - $ 197 $ 1,146 $ 27 Change in unrealized gains or losses included in earnings for derivative contracts held at December 31, 2017: Sales $ - $ - $ - $ - Cost of goods sold - - - - Other income, net 1 (7 ) 18 (3 ) * In January 2017, there was an issuance of a new financial contract (see D11 above). There were no purchases, sales or settlements of Level 3 derivative instruments. Additionally, there were no transfers of derivative instruments into or out of Level 3. Assets Liabilities 2016 Embedded aluminum derivatives Financial contract Embedded aluminum derivatives Embedded credit derivative Financial contract Opening balance—January 1, 2016 $ 1,135 $ 2 $ 169 $ 35 $ 4 Total gains or losses (realized and unrealized) included in: Sales (5 ) - (12 ) - - Cost of goods sold (92 ) - - (5 ) - Other income, net* (13 ) (80 ) 2 5 (2 ) Other comprehensive loss (568 ) 95 47 - (1 ) Purchases, sales, issuances, and settlements** - - 32 - - Transfers into and/or out of Level 3** - - - - - Other 40 - (6 ) - (1 ) Closing balance—December 31, 2016 $ 497 $ 17 $ 232 $ 35 $ - Change in unrealized gains or losses included in earnings for derivative contracts held at December 31, 2016: Sales $ - $ - $ - $ - $ - Cost of goods sold - - - - - Other income, net* (13 ) (80 ) 2 5 (2 ) * In August 2016, Alcoa Corporation elected to terminate the energy contract in accordance with the provisions of the agreement (see D10 above). As a result, Alcoa Corporation decreased the derivative asset and recorded a charge in Other income, net of $84, which is reflected in the table above. Additionally, Alcoa Corporation also decreased the related unrealized gain included in Accumulated other comprehensive loss and recorded a benefit in Other income, net of $84. As such, the termination of the specified term of this derivative contract described above did not have an impact on Alcoa Corporation’s earnings. ** In 2016, there was an issuance of a new embedded derivative contained in an amendment to an existing power contract (see D8 above). There were no purchases, sales or settlements of Level 3 derivative instruments. Additionally, there were no transfers of derivative instruments into or out of Level 3. Derivatives Designated As Hedging Instruments—Cash Flow Hedges For derivative instruments that are designated and qualify as cash flow hedges, the effective portion of unrealized gains or losses on the derivative is reported as a component of other comprehensive income. Realized gains or losses on the derivative are reclassified from other comprehensive income into earnings in the same period or periods during which the hedged transaction impacts earnings. Gains and losses on the derivative representing either hedge ineffectiveness or hedge components excluded from the assessment of effectiveness are recognized directly in earnings immediately. Alcoa Corporation has five Level 3 embedded aluminum derivatives and one Level 3 financial contract (through November 2016 – see D10 above) that have been designated as cash flow hedges as described below. Additionally, in January 2017, Alcoa Corporation entered into a new financial contract, which was designated as a cash flow hedging instrument and was classified as Level 3 under the fair value hierarchy (see D11 above), that replaced the existing financial contract in August 2017. Embedded aluminum derivatives (D1 through D5). LME-linked In 2017, 2016, and 2015, Alcoa Corporation recognized a net unrealized loss of $1,521, a net unrealized loss of $615, and a net unrealized gain of $1,155, respectively, in Other comprehensive loss related to these five derivative instruments. Additionally, Alcoa Corporation reclassified a realized loss of $113, $7, and $21 from Accumulated other comprehensive loss to Sales in 2017, 2016, and 2015, respectively. Assuming market rates remain constant with the rates at December 31, 2017, a realized loss of $120 is expected to be recognized in Sales over the next 12 months. There was no ineffectiveness related to these five derivative instruments in 2017, 2016, and 2015. Financial contracts (D10 and D11). In addition, in January 2017, Alcoa Corporation entered into a new financial contract that hedges the anticipated power requirements at this smelter for the period from August 2017 through July 2021 (see D11 above). At December 31, 2017, this financial contract hedges forecasted electricity purchases of 8,805,456 megawatt hours. In 2017, Alcoa Corporation recognized an unrealized gain of $88 in Other comprehensive loss. Additionally, Alcoa Corporation reclassified a realized gain of $25 in 2017 from Accumulated other comprehensive loss to Cost of goods sold. Assuming market rates remain consistent with the rates at December 31, 2017, a realized gain of $96 is expected to be recognized in Cost of goods sold over the next 12 months. Additionally, Alcoa Corporation recognized a loss of less than $1 in Other income, net related to hedge ineffectiveness in 2017. Derivatives Not Designated As Hedging Instruments Alcoa Corporation has two (three prior to October 2016) Level 3 embedded aluminum derivatives (D6 through D8) and one Level 3 embedded credit derivative (D9) that do not qualify for hedge accounting treatment and one Level 3 financial contract that management elected to discontinue hedge accounting treatment (see D10 above). As such, gains and losses related to the changes in fair value of these instruments are recorded directly in earnings. In 2017, 2016, and 2015, Alcoa Corporation recognized a loss of $22, $17, and $25, respectively, in Other (income) expenses, net, of which a loss of $18, $15, and $8, respectively, related to the embedded aluminum derivatives, a gain of $3, a loss of $5, and a loss of $17, respectively, related to the embedded credit derivative, and, a loss of $7 and a gain of $3 (no amount for 2015), respectively, related to the financial contract. Material Limitations The disclosures with respect to commodity prices and foreign currency exchange risk do not consider the underlying commitments or anticipated transactions. If the underlying items were included in the analysis, the gains or losses on the futures contracts may be offset. Actual results will be determined by several factors that are not under Alcoa Corporation’s control and could vary significantly from those factors disclosed. Alcoa Corporation is exposed to credit loss in the event of nonperformance by counterparties on the above instruments, as well as credit or performance risk with respect to its hedged customers’ commitments. Although nonperformance is possible, Alcoa Corporation does not anticipate nonperformance by any of these parties. Contracts are with creditworthy counterparties and are further supported by cash, treasury bills, or irrevocable letters of credit issued by carefully chosen banks. In addition, various master netting arrangements are in place with counterparties to facilitate settlement of gains and losses on these contracts. Other Financial Instruments. December 31, 2017 2016 Carrying Fair Carrying Fair Cash and cash equivalents $ 1,358 $ 1,358 $ 853 $ 853 Restricted cash 7 7 6 6 Long-term debt due within one year 16 16 21 21 Long-term debt, less amount due within one year 1,388 1,555 1,424 1,573 The following methods were used to estimate the fair values of other financial instruments: Cash and cash equivalents and Restricted cash. Long-term debt due within one year and Long-term debt, less amount due within one year. non-public

Income Taxes

Income Taxes	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Income Taxes	P. Income Taxes Provision for income taxes. 2017 2016 2015 Domestic $ (712 ) $ (688 ) $ (1,053 ) Foreign 1,871 526 716 $ 1,159 $ (162 ) $ (337 ) Provision for income taxes consisted of the following: 2017 2016 2015 Current: Federal* $ 3 $ 9 $ 3 Foreign 421 221 313 State and local - - - 424 230 316 Deferred: Federal* 24 - (85 ) Foreign 152 (46 ) 171 State and local - - - 176 (46 ) 86 Total $ 600 $ 184 $ 402 * Includes U.S. income taxes related to foreign income A reconciliation of the U.S. federal statutory rate to Alcoa Corporation’s effective tax rate was as follows (the effective tax rate was a provision on income in 2017 and a provision on a loss in 2016 and 2015): 2017 2016 2015 U.S. federal statutory rate 35.0 % 35.0 % 35.0 % Changes in valuation allowances 25.8 (1.9 ) (62.6 ) Taxes on foreign operations—rate differential (10.8 ) 44.3 14.2 Impact of U.S. Tax Cuts and Jobs Act of 2017 1.9 - - Noncontrolling interest 1.4 (7.3 ) (8.5 ) Other taxes related to foreign operations 1.3 (19.5 ) (20.9 ) Tax holidays (1) 0.4 11.2 6.2 Losses and credits with no tax benefit (2) (0.2 ) (163.2 ) (82.0 ) Statutory tax rate and law changes 0.1 (0.6 ) (0.3 ) Nondeductible costs related to the Separation Transaction - (9.6 ) - Impact of capitalization of intercompany debt - - 3.3 Other (3.1 ) (2.0 ) (3.7 ) Effective tax rate 51.8 % (113.6 )% (119.3 )% (1) As of December 31, 2017, the income of certain operations of several of the Company’s subsidiaries in Brazil are taxed at a lower rate as a result of approved tax holidays. The difference between the respective holiday rates and the statutory rates resulted in a benefit of $20, or $0.11 per diluted share, in 2017. The majority of these tax holidays expire at the end of 2022 and one tax holiday expires at the end of 2026 (see below). In 2017, this line item also includes a charge of $26 for the remeasurement of certain deferred tax assets at the holiday rate (see below). (2) In 2016 and 2015, hypothetical net operating losses and tax credits determined on a separate return basis for which it is more likely than not that a tax benefit will not be realized. The related deferred tax asset and offsetting valuation allowance have been adjusted to Parent Company net investment and, as such, are not reflected in subsequent deferred tax and valuation allowance tables. In mid-2017, AWAB received approval for a tax holiday related to the operation of the Juruti (Brazil) bauxite mine. This tax holiday is effective as of January 1, 2017 (retroactively) and decreases AWAB’s tax rate on income generated by the Juruti mine from 34% to 15.25%, which will result in future cash tax savings over a 10-year period. As a result of this income tax rate change, AWAB’s existing deferred tax assets that are expected to reverse during the holiday period were remeasured at the lower tax rate. This remeasurement resulted in both a decrease to AWAB’s deferred tax assets and a discrete income tax charge of $26 ($15 after noncontrolling interest). Deferred income taxes. 2017 2016 December 31, Deferred tax assets Deferred tax liabilities Deferred tax assets Deferred tax liabilities Tax loss carryforwards $ 1,185 $ - $ 1,064 $ - Employee benefits 949 - 1,240 - Derivatives and hedging activities 287 70 - 124 Loss provisions 246 - 313 - Tax credit carryforwards 193 - 23 - Depreciation 141 432 187 499 Deferred income/expense 11 109 28 136 Other 100 57 233 125 3,112 668 3,088 884 Valuation allowance (1,927 ) - (1,755 ) - $ 1,185 $ 668 $ 1,333 $ 884 The following table details the expiration periods of the deferred tax assets presented above: December 31, 2017 Expires within 10 years Expires within 11-20 years No expiration* Other* Total Tax loss carryforwards $ 330 $ 250 $ 605 $ - $ 1,185 Tax credit carryforwards 193 - - - 193 Other - - 297 1,437 1,734 Valuation allowance (523 ) (152 ) (315 ) (937 ) (1,927 ) $ - $ 98 $ 587 $ 500 $ 1,185 * Deferred tax assets with no expiration may still have annual limitations on utilization. Other represents deferred tax assets whose expiration is dependent upon the reversal of the underlying temporary difference. The total deferred tax asset (net of valuation allowance) is supported by projections of future taxable income exclusive of reversing temporary differences and taxable temporary differences that reverse within the carryforward period. The composition of Alcoa Corporation’s net deferred tax asset by jurisdiction as of December 31, 2017 was as follows: Domestic Foreign Total Deferred tax assets $ 1,164 $ 1,948 $ 3,112 Valuation allowance (1,050 ) (877 ) (1,927 ) Deferred tax liabilities (108 ) (560 ) (668 ) $ 6 $ 511 $ 517 The Company has several income tax filers in various foreign countries. The $511 net deferred tax asset included under the “Foreign” column in the table above was comprised of the following (by income tax filer): a $252 net deferred tax asset for Alumínio in Brazil; a $153 net deferred tax asset for AWAB in Brazil; a $112 deferred tax asset for Alúmina Española, S.A. (“Española” and collectively with Alumínio and AWAB, the “Foreign Filers”) in Spain; and a combined $6 net deferred tax liability for several other foreign income tax filers. The future realization of the net deferred tax asset for each of the Foreign Filers was based on projections of the respective future taxable income (defined as the sum of pretax income, other comprehensive income, and permanent tax differences), exclusive of reversing temporary differences and carryforwards. The realization of the net deferred tax assets of the Foreign Filers is not dependent on any tax planning strategies. The Foreign Filers each generated taxable income in the three-year cumulative period ending December 31, 2017. Management has also forecasted taxable income for each of the Foreign Filers in 2018 and for the foreseeable future. This forecast is based on macroeconomic indicators and involves assumptions related to, among others: commodity prices; volume levels; and key inputs and raw materials, such as bauxite, caustic soda, alumina, calcined petroleum coke, liquid pitch, energy (fuel oil, natural gas, electricity), labor, and transportation costs. These are the same assumptions used by management to develop a financial and operating plan, which is used to run the Company and measure performance against actual results. The majority of the Foreign Filers’ net deferred tax assets relate to tax loss carryforwards. The Foreign Filers do not have a history of tax loss carryforwards expiring unused. Additionally, tax loss carryforwards have an infinite life under the respective income tax codes in Brazil and Spain. That said, utilization of an existing tax loss carryforward is limited to 30% and 25% of taxable income in a particular year in Brazil and Spain, respectively. Accordingly, management concluded that the net deferred tax assets of the Foreign Filers will more likely than not be realized in future periods, resulting in no need for a partial or full valuation allowance as of December 31, 2017. The following table details the changes in the valuation allowance: December 31, 2017 2016 2015 Balance at beginning of year $ (1,755 ) $ (712 ) $ (486 ) Establishment of new allowances (1) (94 ) - (141 ) Net change to existing allowances (2) (33 ) (1,056 ) (148 ) U.S. state tax apportionment and tax rate changes - - 30 Foreign currency translation (45 ) 13 33 Balance at end of year $ (1,927 ) $ (1,755 ) $ (712 ) (1) This line item reflects valuation allowances initially established as a result of a change in management’s judgment regarding the realizability of deferred tax assets. (2) This line item reflects movements in previously established valuation allowances, which increase or decrease as the related deferred tax assets increase or decrease. Such movements occur as a result of remeasurement due to a tax rate change and changes in the underlying attributes of the deferred tax assets, including expiration of the attribute and reversal of the temporary difference that gave rise to the deferred tax asset. In 2017, Alcoa Corporation established a valuation allowance of $94 related to the remaining deferred tax assets in Iceland (see below). This amount was comprised of a $60 discrete income charge recognized in the Provision for income taxes on the accompanying Consolidated Statement of Operations and a $34 charge to Accumulated other comprehensive loss on the accompanying Consolidated Balance Sheet. These deferred tax assets relate to tax loss carryforwards, which have an expiration period ranging from 2017 to 2026, and deferred losses associated with derivative and hedging activities. After weighing all available positive and negative evidence, management determined that it was no longer more likely than not that Alcoa Corporation will realize the tax benefit of these deferred tax assets. This conclusion was based on existing cumulative losses and a short expiration period. Such losses were generated as a result of intercompany interest expense under the Company’s global treasury and cash management system and the realization of deferred losses associated with an LME-linked embedded derivative in a power contract (see Note O). Such interest expense is expected to continue and additional deferred losses associated with the embedded derivative will be realized in future years. As a result, management estimates that there will not be sufficient taxable income available to utilize the operating losses during the expiration period. The need for this valuation allowance will be assessed on a continuous basis in future periods and, as a result, a portion or all of the allowance may be reversed based on changes in facts and circumstances. In 2015, Alcoa Corporation recognized a $141 discrete income tax charge for valuation allowances on certain deferred tax assets in Suriname and Iceland. Of this amount, an $85 valuation allowance was established on the full value of the deferred tax assets in Suriname. These deferred tax assets relate to tax loss carryforwards and employee benefits have an expiration period ranging from 2016 to 2022. The remaining $56 charge relates to a valuation allowance established on a portion of the deferred tax assets in Iceland, which were related to tax loss carryforwards that have an expiration period ranging from 2017 to 2019. After weighing all available positive and negative evidence, management determined that it was no longer more likely than not that Alcoa Corporation will realize the tax benefit of these deferred tax assets. This conclusion was mainly driven by a decline in the outlook of the former Primary Metals business (refers to the smelting and casting operations included in what is currently the Aluminum segment), combined with prior year cumulative losses and a short expiration period. The need for this valuation allowance will be assessed on a continuous basis in future periods and, as a result, a portion or all of the allowance may be reversed based on changes in facts and circumstances. Undistributed net earnings. Unrecognized tax benefits. A reconciliation of the beginning and ending amount of unrecognized tax benefits (excluding interest and penalties) was as follows: December 31, 2017 2016 2015 Balance at beginning of year $ 23 $ 22 $ 25 Additions for tax positions of the current year 1 3 2 Additions for tax positions of prior years - 1 1 Reductions for tax positions of prior years (5 ) (2 ) - Settlements with tax authorities (6 ) (2 ) (2 ) Expiration of the statute of limitations (3 ) - - Foreign currency translation - 1 (4 ) Balance at end of year $ 10 $ 23 $ 22 For all periods presented, a portion of the balance at end of year pertains to state tax liabilities, which are presented before any offset for federal tax benefits. The effect of unrecognized tax benefits, if recorded, that would impact the annual effective tax rate for 2017, 2016, and 2015 would be 1%, 10%, and 4%, respectively, of pretax book income (loss). Alcoa Corporation does not anticipate that changes in its unrecognized tax benefits will have a material impact on the Statement of Consolidated Operations during 2018 (see Tax (Spain) in Note R for a matter for which no reserve has been recognized). It is Alcoa Corporation’s policy to recognize interest and penalties related to income taxes as a component of the Provision for income taxes on the accompanying Statement of Consolidated Operations. In 2017, 2016, and 2015, Alcoa Corporation recognized $1, $1, and $7, respectively, in interest and penalties. Due to the expiration of the statute of limitations, settlements with tax authorities, and refunded overpayments, Alcoa Corporation also recognized interest income of $6, $2, and $1 in 2017, 2016, and 2015, respectively. As of December 31, 2017 and 2016, the amount accrued for the payment of interest and penalties was $2 and $6, respectively. U.S. Tax Cuts and Jobs Act of 2017. As a result of the close proximity of the enactment date of the TCJA in relation to the Company’s calendar year-end, management elected January 17, 2018 as a cut-off date for purposes of recognizing any impacts from the TCJA in Alcoa Corporation’s 2017 Consolidated Financial Statements. This date coincided with the Company’s public release of its preliminary financial results for the fourth quarter and year ended December 31, 2017. Accordingly, the Company’s preliminary analysis of the provisions of the TCJA resulted in a charge of $22, which was reflected in the Provision for income taxes on the accompanying Statement of Consolidated Operations for 2017, as described below. The Company expects to finalize its analyses of the TCJA provisions in 2018. The $22 charge relates specifically to management’s reasonable estimate of the corporate income tax rate change, which resulted in the remeasurement of the Company’s deferred income tax accounts. On a gross basis, the Company reduced its deferred tax assets, valuation allowance, and deferred tax liabilities by $506, $433, and $51, respectively. Management also completed a preliminary analysis of the remaining provisions of the TCJA, including those specifically described above, in order to make a reasonable estimate as of the cut-off date, which resulted in no additional impact to the Company’s 2017 Consolidated Financial Statements. Specifically, the reasonable estimate for the TCJA provisions described above was based on the following: for (i), (ii), and (iii), the Company’s existing tax profile, which includes significant current U.S. tax losses that would be applied against such taxable income, as well as significant foreign tax credits that can be applied against these taxes; for (iv) management estimates that the Company will be under the 3% threshold; and for (v) the Company’s deferred income tax assets related to U.S. net operating losses are fully reserved as of December 31, 2017. The Company’s preliminary analyses and provisional estimates of the financial statement impacts of the TCJA were completed in accordance with guidance issued by the SEC under Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act

Asset Retirement Obligations

Asset Retirement Obligations	12 Months Ended
	Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]
Asset Retirement Obligations	Q. Asset Retirement Obligations Alcoa Corporation has recorded AROs related to legal obligations associated with the standard operation of bauxite mines, alumina refineries, and aluminum smelters. These AROs consist primarily of costs associated with mine reclamation, closure of bauxite residue areas, spent pot lining disposal, and landfill closure. Alcoa Corporation also recognizes AROs for any significant lease restoration obligation, if required by a lease agreement, and for the disposal of regulated waste materials related to the demolition of certain power facilities. In addition to AROs, certain CAROs related to alumina refineries, aluminum smelters, rolling mills, and energy generation facilities have not been recorded in the Consolidated Financial Statements due to uncertainties surrounding the ultimate settlement date. Such uncertainties exist as a result of the perpetual nature of the structures, maintenance and upgrade programs, and other factors. At the date a reasonable estimate of the ultimate settlement date can be made (e.g., planned demolition), Alcoa Corporation would record an ARO for the removal, treatment, transportation, storage, and/or disposal of various regulated assets and hazardous materials such as asbestos, underground and aboveground storage tanks, PCBs, various process residuals, solid wastes, electronic equipment waste, and various other materials. If Alcoa Corporation was required to demolish all such structures immediately, the estimated CARO as of December 31, 2017 ranges from $3 to $28 per structure (24 structures) in today’s dollars. The following table details the carrying value of recorded AROs by major category (of which $108 and $104 was classified as a current liability as of December 31, 2017 and 2016, respectively): December 31, 2017 2016 Mine reclamation $ 221 $ 199 Closure of bauxite residue areas 238 219 Spent pot lining disposal 125 135 Demolition* 113 121 Landfill closure 27 30 Other 1 4 $ 725 $ 708 * In 2017, 2016, and 2015, AROs were recorded as a result of management’s decision to permanently close and demolish certain structures (see Note D). The following table details the changes in the total carrying value of recorded AROs: December 31, 2017 2016 Balance at beginning of year $ 708 $ 635 Accretion expense 17 19 Payments (69 ) (47 ) Liabilities incurred 43 117 Foreign currency translation and other 26 (16 ) Balance at end of year $ 725 $ 708

Contingencies and Commitments

Contingencies and Commitments	12 Months Ended
	Dec. 31, 2017
Commitments and Contingencies Disclosure [Abstract]
Contingencies and Commitments	R. Contingencies and Commitments Unless specifically described to the contrary, all matters within Note R are the full responsibility of Alcoa Corporation pursuant to the Separation and Distribution Agreement. Additionally, the Separation and Distribution Agreement provides for cross-indemnities between the Company and Arconic for claims subject to indemnification. Contingencies Litigation. Glencore Italy 148 In December 2017, through an agreement with Invitalia, which is an Italian government agency responsible for managing economic development, Alcoa Corporation settled this matter with CCSE and the Energy Authority for $18 (€15) (paid in January 2018). Accordingly, the Company recorded a reduction of $22 (€19) (the U.S. dollar amount reflects the effects of foreign currency movements since 2015) to its previously established reserve in Restructuring and other charges (see Note D) on the accompanying Statement of Consolidated Operations. In January 2018, subsequent to making the previously referenced payment, Alcoa Corporation and the respective state attorney in Italy filed a joint request with the Administrative Court to have this matter formally dismissed, for which the parties are awaiting notice. Additional terms of the agreement include the transfer of ownership of the Portovesme smelter (permanently closed in 2014) from the Company to Invitalia and the settlement of a groundwater remediation project (see Fusina and Portovesme, Italy in Environmental Matters below). Also, Alcoa will retain previously accrued asset retirement obligations related to ParentCo’s previous decision to demolish the Portovesme smelter; however, such liabilities may be reduced upon Invitalia achieving certain milestones related to the future of the smelter. The carrying value of the assets related to the Portovesme site were previously written down to zero as a result of ParentCo’s decision in 2014 to permanently close the smelter. European Commission Matter. On November 19, 2009, the EC announced a decision in this matter stating that the extension of the tariff by Italy constituted unlawful state aid, in part, and, therefore, the Italian Government is to recover a portion of the benefit ParentCo received since January 2006 (including interest). The amount of this recovery was to be based on a calculation prepared by the Italian Government (see below). In late 2009, after discussions with legal counsel and reviewing the bases on which the EC decided, including the different considerations cited in the EC decision regarding ParentCo’s two smelters in Italy, ParentCo recorded a charge of $250 (€173), which included $20 (€14) to write off a receivable from the Italian Government for amounts due under the now expired tariff structure and $230 (€159) to establish a reserve. On April 19, 2010, ParentCo filed an appeal of this decision with the General Court of the EU (see below). Prior to 2012, ParentCo was involved in other legal proceedings related to this matter that separately sought the annulment of the EC’s July 2006 decision to open an investigation alleging that such decision did not follow the applicable procedural rules and requested injunctive relief to suspend the effectiveness of the EC’s November 19, 2009 decision. However, the decisions by the General Court, and subsequent appeals to the European Court of Justice, resulted in the denial of these remedies. In June 2012, ParentCo received formal notification from the Italian Government with a calculated recovery amount of $375 (€303); this amount was reduced by $65 (€53) for amounts owed by the Italian Government to ParentCo, resulting in a net payment request of $310 (€250). In a notice published in the Official Journal of the European Union on September 22, 2012, the EC announced that it had filed an action against the Italian Government on July 18, 2012 to compel it to collect the recovery amount (on October 17, 2013, the European Court of Justice ordered Italy to so collect). On September 27, 2012, ParentCo received a request for payment in full of the $310 (€250) by October 31, 2012. Following discussions with the Italian Government regarding the timing of such payment, ParentCo paid the requested amount in five quarterly installments of $69 (€50) beginning in October 2012 through December 2013. On October 16, 2014, ParentCo received notice from the General Court of the EU that its April 19, 2010 appeal of the EC’s November 19, 2009 decision was denied. On December 27, 2014, ParentCo filed an appeal of the General Court’s October 16, 2014 ruling to the European Court of Justice (ECJ). Following submission of the EC’s response to the appeal, on June 10, 2015, ParentCo filed a request for an oral hearing before the ECJ; no decision on that request was received. On January 26, 2016, ParentCo was informed that the ECJ had dismissed ParentCo’s December 27, 2014 appeal of the General Court’s October 16, 2014 ruling. The dismissal of ParentCo’s appeal represents the conclusion of the legal proceedings in this matter. Prior to this dismissal, ParentCo had a noncurrent asset of $100 (€91) reflecting the excess of the total of the five payments made to the Italian Government over the reserve recorded in 2009. As a result, this noncurrent asset, along with the $58 (€53) for amounts owed by the Italian Government to ParentCo mentioned above plus $6 (€6) for interest previously paid, was written-off. As a result of the EC’s November 19, 2009 decision, ParentCo’s management had contemplated ceasing operations at its Italian smelters due to uneconomical power costs. In February 2010, ParentCo’s management agreed to continue to operate its smelters in Italy for up to six months while a long-term solution to address increased power costs could be negotiated. Over a portion of this time, a long-term solution was not able to be reached related to the Fusina smelter, therefore, in May 2010, ParentCo and the Italian Government agreed to a temporary idling of the Fusina smelter. As of September 30, 2010, the Fusina smelter was fully curtailed (44,000 metric-tons-per-year). At the end of 2011, as part of a restructuring of ParentCo’s global smelting system, ParentCo’s management decided to curtail operations at the Portovesme smelter during 2012 due to the uncertain prospects for viable, long-term power, along with rising raw materials costs and falling global aluminum prices (mid-2011 metric-tons-per-year). In June 2013 and August 2014, ParentCo decided to permanently shut down and demolish the Fusina and Portovesme smelters, respectively, due to persistent uneconomical conditions. Environmental Matters. A liability is recorded for environmental remediation when a cleanup program becomes probable and the costs can be reasonably estimated. As assessments and cleanups proceed, the liability is adjusted based on progress made in determining the extent of remedial actions and related costs. The liability can change substantially due to factors such as, among others, the nature and extent of contamination, changes in remedial requirements, and technological changes. Alcoa Corporation’s remediation reserve balance was $294 and $324 at December 31, 2017 and 2016 (of which $36 and $60 was classified as a current liability), respectively, and reflects the most probable costs to remediate identified environmental conditions for which costs can be reasonably estimated. In 2017, the remediation reserve was increased by $1 due to a charge of $8 related to the planned demolition of the Rockdale smelter (see Note D), a combined reduction of $6 related to the Baie Comeau and Mosjøen locations (see below), a reversal of $4 related to the restart of the Warrick smelter (see Note D), and a net charge of $3 associated with several other sites. In 2016, the remediation reserve was increased by $39 due to a charge of $26 related to the planned demolition of the Suriname refinery and permanent closure of the related bauxite mines (see Note D) and a net charge of $13 associated with several other sites. In 2015, the remediation reserve was increased by $107 due to a charge of $52 related to the planned demolition of the remaining structures at the Massena East smelter location (see Note D), a charge of $29 related to the planned demolition of the Poços de Caldas smelter and the Anglesea power station (see Note D), a charge of $12 related to the Mosjøen location (see below), a charge of $7 related to the Portovesme location (see below), and a net charge of $7 associated with several other sites. Of the changes to the remediation reserve in 2017, 2016, and 2015, $4, $26, and $86, respectively, was recorded in Restructuring and other charges, while the remainder was recorded in Cost of goods sold on the accompanying Statement of Consolidated Operations. Payments related to remediation expenses applied against the reserve were $48, $32, and $24 in 2017, 2016, and 2015, respectively. These amounts include expenditures currently mandated, as well as those not required by any regulatory authority or third party. In 2017, the change in the reserve also reflects an increase of $17, including $11 due to the effects of foreign currency translation and $5 for the reclassification of an amount previously included in Asset retirement obligations on Alcoa Corporation’s Consolidated Balance Sheet as of December 31, 2016. In 2016, the change in the reserve also reflects an increase for each of the following: $60 of obligations transferred from ParentCo in connection with the Separation Transaction on November 1, 2016, including Sherwin and East St. Louis described below; $17 for the reclassification of amounts included in other reserves within Other noncurrent liabilities and deferred credits on Alcoa Corporation’s Consolidated Balance Sheet as of December 31, 2015; and $5 due to the effects of foreign currency translation. In 2015, the change in the reserve also reflects a decrease of $13 due to the effects of foreign currency translation. The Separation and Distribution Agreement includes provisions for the assignment or allocation of environmental liabilities between Alcoa Corporation and Arconic, including certain remediation obligations associated with environmental matters. In general, the respective parties are responsible for the environmental matters associated with their operations, and with the properties and other assets assigned to each. Additionally, the Separation and Distribution Agreement lists environmental matters with a shared responsibility between the two companies with an allocation of responsibility and the lead party responsible for management of each matter. For matters assigned to Alcoa Corporation and Arconic under the Separation and Distribution Agreement, Alcoa Corporation and Arconic have agreed to indemnify Arconic and Alcoa Corporation in whole or in part for environmental liabilities arising from operations prior to the Separation Date. The following description provides details regarding the current status of certain significant reserves related to current or former Alcoa Corporation sites. With the exception of the Fusina, Italy matter, Alcoa Corporation assumed full responsibility of the matters described below. General Sherwin, TX Baie Comeau, Quebec, Canada Fusina and Portovesme, Italy— pre-existing clean-up In December 2009, Trasformazioni and Ligestra reached an initial agreement for settlement of the liabilities related to the Fusina operations while negotiations continued related to Portovesme (see below). The agreement outlined an allocation of payments to the MOE for emergency action and natural resource damages and the scope and costs for a proposed soil remediation project, which was formally presented to the MOE in mid-2010. In January 2014, in anticipation of ParentCo reaching a final administrative agreement with the MOE, ParentCo and Ligestra entered into a final agreement related to Fusina for allocation of payments to the MOE for emergency action and natural resource damages and the costs for the approved soil remediation project. The agreement resulted in Ligestra assuming 50% to 80% of all payments and remediation costs. On February 27, 2014, ParentCo and the MOE reached a final administrative agreement for conduct of work. The agreement includes both a soil and groundwater remediation project estimated to cost $33 (€24) and requires payments of $25 (€18) to the MOE for emergency action and natural resource damages. Based on the final agreement with Ligestra, ParentCo’s share of all costs and payments was $17 (€12), of which $9 (€6) related to the damages will be paid annually over a 10-year Effective with the Separation Transaction, Arconic retained the portion of this obligation related to the Fusina rolling operations. Specifically, under the Separation and Distribution Agreement, Trasformazioni, and with it the Fusina properties, were assigned to Alcoa Corporation. Fusina Rolling S.r.l., entered into a lease agreement for the portion of property that included the rolling operation. Pursuant to the Separation and Distribution Agreement, the liabilities at Fusina described above were allocated between Alcoa Corporation (Trasformazioni) and Arconic (Fusina Rolling S.r.l.). Arconic will pay $7 (€7) for the portion of remediation expenses associated with the section of property that includes the rolling operation as the project is completed. Separately, in 2009, due to additional information derived from the site investigations conducted at Portovesme, ParentCo increased the reserve by $3. In November 2011, Trasformazioni and Ligestra reached an agreement for settlement of the liabilities related to Portovesme, similar to the one for Fusina. A proposed soil remediation project for Portovesme was formally presented to the MOE in June 2012. Neither the agreement with Ligestra nor the proposal to the MOE resulted in a change to the reserve for Portovesme. In November 2013, the MOE rejected the proposed soil remediation project and requested a revised project be submitted. In May 2014, Trasformazioni and Ligestra submitted a revised soil remediation project that addressed certain stakeholders’ concerns. ParentCo increased the reserve by $3 in 2014 to reflect the estimated higher costs associated with the revised soil remediation project, as well as current operating and maintenance costs of the Portovesme site. In October 2014, the MOE required a further revised project be submitted to reflect the removal of a larger volume of contaminated soil than what had been proposed, as well as design changes for the cap related to the remaining contaminated soil left in place and the expansion of an emergency containment groundwater pump and treatment system that was previously installed. Trasformazioni and Ligestra submitted the further revised soil remediation project in February 2015. As a result, ParentCo increased the reserve by $7 in March 2015 to reflect the increase in the estimated costs of the project. In October 2015, ParentCo received a final ministerial decree approving the February 2015 revised soil remediation project. Work on the soil remediation project commenced in mid-2016 Mosjøen, Norway In April 2015, the NEA notified ParentCo that the revised project was approved and required submission of the final project design before issuing a final order. ParentCo completed and submitted the final project design, which identified a need to stabilize the related wharf structure to allow for the sediment dredging in the harbor. As a result, ParentCo increased the reserve for Mosjøen by $11 in June 2015 to reflect the estimated cost of the wharf stabilization. Also in June 2015, the NEA issued a final order approving the project as well as the final project design. In September 2015, ParentCo increased the reserve by $1 to reflect the potential need (based on prior experience with similar projects) to perform additional dredging if the results of sampling, which is required by the order, don’t achieve the required cleanup levels. Project construction commenced in early 2016 and is expected to be completed in 2018. In the second half of 2017, Alcoa Corporation reduced the reserve associated with this matter by $2 based on a revised cost estimate of the remaining project work. At December 31, 2017 and 2016, the reserve balance associated with this matter was $2 and $8, respectively. East St. Louis, IL Tax. Spain Additionally, following a corporate income tax audit of the same Spanish tax group for the 2006 through 2009 tax years, Spain’s tax authorities issued an assessment in July 2013 similarly disallowing certain interest deductions. In August 2013, ParentCo filed an appeal of this second assessment in Spain’s Central Tax Administrative Court, which was denied in January 2015. ParentCo filed an appeal of this second assessment in Spain’s National Court in March 2015. On January 16, 2017, Spain’s National Court issued a decision in favor of the Company related to the assessment received in September 2010. On March 6, 2017, the Company was notified that Spain’s tax authorities did not file an appeal, for which the deadline has passed. As a result, the assessment related to the 2003 through 2005 tax years is null and void. Spain’s National Court has not yet rendered a decision related to the assessment received in July 2013 for the 2006 through 2009 tax years. The amount of this assessment on a standalone basis, including interest, was $155 (€130) as of December 31, 2017. The Company believes it has meritorious arguments to support its tax position and intends to vigorously litigate the remaining assessment through Spain’s court system. However, in the event the Company is unsuccessful, a portion of the remaining assessment may be offset with existing tax loss carryforwards available to the Spanish consolidated tax group, which would be shared between the Company and Arconic as provided for in the Tax Matters Agreement related to the Separation Transaction. Additionally, it is possible that the Company may receive similar assessments for tax years subsequent to 2009 (see below). Despite the favorable decision received on the first assessment, at this time, the Company is unable to reasonably predict the ultimate outcome for this matter. This Spanish consolidated tax group had been under audit (began in September 2015) for the 2010 through 2013 tax years. As Spain’s tax authorities neared the end of the audit, they informed both Alcoa Corporation and Arconic of their intent to issue an assessment similarly disallowing certain interest deductions as the two assessments described above. On August 3, 2017, in lieu of receiving a formal assessment, all parties agreed to a settlement related to the 2010 through 2013 tax years. For Alcoa Corporation, this settlement is not material to the Company’s Consolidated Financial Statements. Given the stage of the appeal of the assessment for the 2006 through 2009 tax years in Spain’s National Court, the settlement of the 2010 through 2013 tax years will not impact the ultimate outcome of that proceeding. Brazil (AWAB) Brazil (Alumínio) Other. In 2000, ParentCo acquired Reynolds Metals Company (“Reynolds,” a subsidiary of Alcoa Corporation), which included an alumina refinery in Gregory, Texas. As a condition of the Reynolds acquisition, ParentCo was required to divest this alumina refinery. In accordance with the terms of the divestiture in 2000, ParentCo agreed to retain responsibility for certain environmental obligations (see Environmental Matters above) and assigned to the buyer an Energy Services Agreement (“ESA”) with Gregory Power Partners (“Gregory Power”) for purchase of steam and electricity by the refinery. As a result of Sherwin’s initial bankruptcy filing, separate legal actions were initiated against Reynolds by Gregory Power and Sherwin as follows. Gregory Power— pre-trial Sherwin— General. Commitments Purchase Obligations. On April 8, 2015, AofA secured a new 12-year Operating Leases. Guarantees of Third Parties. Bank Guarantees and Letters of Credit. one-year In August 2017, Alcoa Corporation entered into a one-year Surety Bonds.

Other Financial Information

Other Financial Information	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Other Financial Information	S. Other Financial Information Interest Cost Components 2017 2016 2015 Amount charged to expense $ 104 $ 243 $ 270 Amount capitalized 17 23 30 $ 121 $ 266 $ 300 Other (Income) Expenses, Net 2017 2016 2015 Equity loss $ 28 $ 70 $ 89 Foreign currency losses (gains), net 8 8 (39 ) Net gain from asset sales (116 ) (164 ) (32 ) Net loss on mark-to-market 24 9 26 Other, net (2 ) (12 ) (2 ) $ (58 ) $ (89 ) $ 42 In 2017, Net gain from asset sales included a $122 gain related to the sale of Yadkin (see Note C). In 2016, Net gain from asset sales included a $118 gain related to the sale of wharf property near the Intalco (Washington) smelter and a $27 gain related to the sale of an equity interest in a natural gas pipeline in Australia (see Note H). In 2015, Net gain from asset sales included a $29 gain related to the sale of land around the Lake Charles, Louisiana anode facility. Other Noncurrent Assets December 31, 2017 2016 Gas supply prepayment (R) $ 510 $ 471 Value-added tax credits (1) 340 287 Prepaid gas transmission contract (H) 300 270 Goodwill (K) 154 155 Deferred mining costs, net (2) 139 127 Prepaid pension benefit (N) 72 43 Intangibles, net (K) 62 135 Other 142 180 $ 1,719 $ 1,668 (1) The Value-added tax credits relate to two of the Company’s subsidiaries in Brazil, AWAB and Alumínio. (2) As of December 31, 2016, this amount reflects an asset impairment of $72 (see Note D). Other Noncurrent Liabilities and Deferred Credits December 31, 2017 2016 Accrued compensation and retirement costs $ 127 $ 122 Deferred alumina sales revenue 68 76 Liability related to the resolution of a legal matter* - 74 Other 84 98 $ 279 $ 370 * In early 2014, ParentCo and one of Alcoa’s Corporation’s current subsidiaries, AWA, resolved violations of certain provisions of the Foreign Corrupt Practices Act of 1977 with the U.S. Department of Justice and U.S. Securities and Exchange Commission. Under the resolution, ParentCo and AWA agreed to pay a combined $384 over a four-year timeframe. Prior to the Separation Transaction, ParentCo and AWA paid $236 of the total amount. As part of the Separation and Distribution Agreement, Alcoa Corporation assumed ParentCo’s portion of the $148 remaining obligation. The $148 was paid in equal installments of $74 in January 2017 and January 2018. Cash Flow Information Cash paid for interest and income taxes was as follows: 2017 2016 2015 Interest, net of amount capitalized $ 100 $ 226 $ 270 Income taxes, net of amount refunded 363 265 265 Noncash Financing and Investing Activities.

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2017
Subsequent Events [Abstract]
Subsequent Events	T. Subsequent Events Management evaluated all activity of Alcoa Corporation and concluded that no subsequent events have occurred that would require recognition in the Consolidated Financial Statements or disclosure in the Notes to the Consolidated Financial Statements, except as described below. On January 17, 2018, the Company communicated retirement benefit changes to certain U.S. and Canadian employees. Effective January 1, 2021, all salaried U.S. and Canadian employees that are participants in the Company’s defined benefit pension plans will cease accruing retirement benefits for future service. This change will affect approximately 800 employees, who will be transitioned to country-specific defined contribution plans, in which the Company will contribute 3% of these participants’ eligible earnings on an annual basis. Such contributions will be incremental to any employer savings match the employees may receive under existing defined contribution plans. Participants already collecting benefits under the defined benefit pension plans, as well as those currently covered by collective bargaining agreements, are not affected by these changes. Also, effective January 1, 2021, Alcoa Corporation will no longer contribute to pre-Medicare Separately, Alcoa expects to make discretionary contributions of approximately $300 combined to the U.S. and Canadian defined benefit pension plans in 2018. The Company intends these contributions to be used to purchase annuity contracts for approximately 9,000 retirees. Such instruments will allow the Company to transfer risk and lower its costs related to these defined benefit pension plans. At such time(s) annuity contracts are purchased, Alcoa Corporation will record a settlement charge as part of the net periodic benefit cost of these defined benefit pension plans.

Basis of Presentation (Policies

Basis of Presentation (Policies)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Separation Transaction	Separation Transaction. The Separation Transaction was subject to a number of conditions, including, but not limited to: final approval by ParentCo’s Board of Directors (see below); the continuing validity of the private letter ruling from the Internal Revenue Service regarding certain U.S. federal income tax matters relating to the transaction; receipt of an opinion of legal counsel regarding the qualification of the distribution, together with certain related transactions, as a transaction that is generally tax-free On September 29, 2016, ParentCo’s Board of Directors approved the completion of the Separation Transaction by means of a pro rata distribution by ParentCo of 80.1% of the outstanding common stock of Alcoa Corporation to ParentCo shareholders of record as of the close of business on October 20, 2016 (the “Record Date”). Arconic was to retain the remaining 19.9% of Alcoa Corporation common stock. At the time of the Separation Transaction, ParentCo shareholders were to receive one share of Alcoa Corporation common stock for every three shares of ParentCo common stock held as of the close of business on the Record Date. ParentCo shareholders were to receive cash in lieu of fractional shares. In connection with the Separation Transaction, as of October 31, 2016, Alcoa Corporation entered into certain agreements with Arconic to implement the legal and structural separation between the two companies, govern the relationship between Alcoa Corporation and Arconic after the completion of the Separation Transaction, and allocate between Alcoa Corporation and Arconic various assets, liabilities and obligations, including, among other things, employee benefits, environmental liabilities, intellectual property, and tax-related Know-How, On November 1, 2016 (the “Separation Date”), the Separation Transaction was completed and became effective at 12:01 a.m. Eastern Standard Time. To effect the Separation Transaction, ParentCo undertook a series of transactions to separate the net assets and certain legal entities of ParentCo, resulting in a cash payment of $1,072 to ParentCo by Alcoa Corporation (an additional $247 was paid to Arconic by Alcoa Corporation in 2017, including $243 associated with the sale of certain of the Company’s energy operations —see Note C) with the net proceeds of a previous debt offering (see Note L). In conjunction with the Separation Transaction, 146,159,428 shares of Alcoa Corporation common stock were distributed to ParentCo shareholders. Additionally, Arconic retained 36,311,767 shares of Alcoa Corporation common stock representing its 19.9% retained interest (Arconic sold all of these shares in 2017). “Regular-way” ParentCo incurred costs to evaluate, plan, and execute the Separation Transaction, and Alcoa Corporation was allocated a pro rata portion of those costs based on segment revenue (see Cost Allocations below). ParentCo recognized $152 from January 2016 through October 2016 and $24 in 2015 for costs related to the Separation Transaction, of which $68 and $12, respectively, was allocated to Alcoa Corporation. The allocated amounts were included in Selling, general administrative, and other expenses on the accompanying Statement of Consolidated Operations.
Basis of Presentation	Basis of Presentation.
Principles of Consolidation	Principles of Consolidation. AWAC is an unincorporated global joint venture between Alcoa Corporation and Alumina Limited of Australia (Alumina Limited) and consists of several affiliated operating entities, which own, or have an interest in, or operate the bauxite mines and alumina refineries within Alcoa Corporation’s Bauxite and Alumina segments (except for the Poços de Caldas mine and refinery and a portion of the São Luís refinery, all in Brazil) and the Portland smelter in Australia. Alcoa Corporation owns 60% and Alumina Limited owns 40% of these individual entities, which are consolidated by the Company for financial reporting purposes and include Alcoa of Australia Limited (AofA), Alcoa World Alumina LLC (AWA), and Alcoa World Alumina Brasil Ltda. (AWAB). Alumina Limited’s interest in the equity of such entities is reflected as Noncontrolling interest on the accompanying Consolidated Balance Sheet. In 2017, 2016, and 2015, AWAC received $80, $48, and $2, respectively, in contributions from Alumina Limited. Management evaluates whether an Alcoa Corporation entity or interest is a variable interest entity and whether Alcoa Corporation is the primary beneficiary. Consolidation is required if both of these criteria are met. Alcoa Corporation does not have any variable interest entities requiring consolidation. Prior to the Separation Date, Alcoa Corporation did not operate as a separate, standalone entity. Alcoa Corporation’s operations were included in ParentCo’s financial results. Accordingly, for all periods prior to the Separation Date, the accompanying Consolidated Financial Statements were prepared from ParentCo’s historical accounting records and were presented on a standalone basis as if Alcoa Corporation’s operations had been conducted independently from ParentCo. Such Consolidated Financial Statements include the historical operations that were considered to comprise Alcoa Corporation’s businesses, as well as certain assets and liabilities that were historically held at ParentCo’s corporate level but were specifically identifiable or otherwise attributable to Alcoa Corporation. ParentCo’s net investment in these operations is reflected as Parent Company net investment on the accompanying Consolidated Financial Statements. All significant transactions and accounts within Alcoa Corporation have been eliminated. All significant intercompany transactions between ParentCo and Alcoa Corporation were included within Parent Company net investment on the accompanying Consolidated Financial Statements.
Cost Allocations	Cost Allocations. The Consolidated Financial Statements of Alcoa Corporation include general corporate expenses of ParentCo that were not historically charged to Alcoa Corporation for certain support functions that were provided on a centralized basis, such as expenses related to finance, audit, legal, information technology, human resources, communications, compliance, facilities, employee benefits and compensation, and research and development activities. These general corporate expenses were included on the accompanying Statement of Consolidated Operations within Cost of goods sold, Selling, general administrative and other expenses, and Research and development expenses. These expenses were allocated to Alcoa Corporation on the basis of direct usage when identifiable, with the remainder allocated based on Alcoa Corporation’s segment revenue as a percentage of ParentCo’s total segment revenue for both Alcoa Corporation and Arconic. All external debt not directly attributable to Alcoa Corporation was excluded from Alcoa Corporation’s Consolidated Balance Sheet. Financing costs related to these debt obligations were allocated to Alcoa Corporation based on the ratio of capital invested in Alcoa Corporation to the total capital invested by ParentCo in both Alcoa Corporation and Arconic, and were included on the accompanying Statement of Consolidated Operations within Interest expense. The following table reflects the allocations described above: 2016 2015 Cost of goods sold (1) $ 40 $ 93 Selling, general administrative, and other expenses (2) 150 146 Research and development expenses 2 17 Provision for depreciation, depletion, and amortization 18 22 Restructuring and other charges (3) 1 32 Interest expense 198 245 Other (income) expenses, net $ (7 ) $ 12 (1) Allocation principally relates to expenses for ParentCo’s retained pension and other postretirement benefits associated with closed and sold operations. (2) Allocation includes costs incurred by ParentCo associated with the Separation Transaction (see Separation Transaction above). (3) Allocation primarily relates to layoff programs for ParentCo corporate employees. Management believes the assumptions regarding the allocation of ParentCo’s general corporate expenses and financing costs were reasonable. Nevertheless, the Consolidated Financial Statements of Alcoa Corporation may not include all of the actual expenses that would have been incurred and may not reflect Alcoa Corporation’s consolidated results of operations, financial position, and cash flows had it been a standalone company during the periods prior to the Separation Date. Actual costs that would have been incurred if Alcoa Corporation had been a standalone company would depend on multiple factors, including organizational structure, capital structure, and strategic decisions made in various areas, including information technology and infrastructure. Transactions between Alcoa Corporation and ParentCo, including sales to Arconic, were included as related party transactions on the Consolidated Financial Statements and are considered to be effectively settled for cash at the time the transaction was recorded. The total net effect of the settlement of these transactions is reflected on the accompanying Statement of Consolidated Cash Flows as a financing activity and on Alcoa Corporation’s Consolidated Balance Sheet as Parent Company net investment.
Cash Management	Cash Management. Cash was managed centrally with certain net earnings reinvested locally and working capital requirements met from existing liquid funds. Accordingly, the cash and cash equivalents held by ParentCo at the corporate level were not attributed to Alcoa Corporation for any of the periods prior to the Separation Date. Only cash amounts specifically attributable to Alcoa Corporation were reflected in the Company’s Consolidated Balance Sheet. Transfers of cash, both to and from ParentCo’s centralized cash management system, were reflected as a component of Parent Company net investment on Alcoa Corporation’s Consolidated Balance Sheet and as a financing activity on the accompanying Consolidated Statement of Cash Flows. ParentCo had an arrangement with several financial institutions to sell certain customer receivables without recourse on a revolving basis. The sale of such receivables was completed through the use of a bankruptcy-remote special-purpose entity, which was a consolidated subsidiary of ParentCo. In connection with this arrangement, certain of Alcoa Corporation’s customer receivables were sold on a revolving basis to this bankruptcy-remote subsidiary of ParentCo; these sales were reflected as a component of Parent Company net investment on Alcoa Corporation’s Consolidated Balance Sheet. ParentCo participated in several accounts payable settlement arrangements with certain vendors and third-party intermediaries. These arrangements provided that, at the vendor’s request, the third-party intermediary advance the amount of the scheduled payment to the vendor, less an appropriate discount, before the scheduled payment date and ParentCo made payment to the third-party intermediary on the date stipulated in accordance with the commercial terms negotiated with its vendors. In connection with these arrangements, certain of Alcoa Corporation’s accounts payable were settled, at the vendor’s request, before the scheduled payment date; these settlements were reflected as a component of Parent Company net investment on Alcoa Corporation’s Consolidated Balance Sheet.
Related Party Transactions	Related Party Transactions. Transactions between Alcoa Corporation and Arconic have been presented as related party transactions on the accompanying Consolidated Financial Statements. Sales to Arconic from Alcoa Corporation were $864, $958, and $1,078 in 2017, 2016, and 2015, respectively. As of December 31, 2017 and 2016, outstanding receivables from Arconic were $84 and $67, respectively, and were included in Receivables from customers on the accompanying Consolidated Balance Sheet.
Cash Equivalents	Cash Equivalents.
Inventory Valuation	Inventory Valuation. last-in, first-out
Properties, Plants, and Equipment	Properties, Plants, and Equipment. one-year) ramp-up Segment Structures Machinery and equipment Bauxite mining 35 17 Alumina refining 30 28 Aluminum smelting and casting 36 22 Energy generation 33 25 Aluminum rolling 31 17 Repairs and maintenance are charged to expense as incurred. Gains or losses from the sale of assets are generally recorded in Other (income) expenses, net. Properties, plants, and equipment are reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of such assets (asset group) may not be recoverable. Recoverability of assets is determined by comparing the estimated undiscounted net cash flows of the operations related to the assets (asset group) to their carrying amount. An impairment loss would be recognized when the carrying amount of the assets (asset group) exceeds the estimated undiscounted net cash flows. The amount of the impairment loss to be recorded is calculated as the excess of the carrying value of the assets (asset group) over their fair value, with fair value determined using the best information available, which generally is a discounted cash flow (DCF) model. The determination of what constitutes an asset group, the associated estimated undiscounted net cash flows, and the estimated useful lives of assets also require significant judgments
Equity Investments	Equity Investments.
Deferred Mining Costs	Deferred Mining Costs.
Goodwill and Other Intangible Assets	Goodwill and Other Intangible Assets. Goodwill is allocated among and evaluated for impairment at the reporting unit level, which is defined as an operating segment or one level below an operating segment. Alcoa Corporation has five reporting units, of which three are included in the Aluminum segment (smelting/casting, energy generation, and rolling operations). The remaining two reporting units are the Bauxite and Alumina segments. Of these five reporting units, only Bauxite and Alumina contain goodwill. As of December 31, 2017, the carrying value of the goodwill for Bauxite and Alumina was $51 and $103, respectively. These amounts include an allocation of goodwill held at the corporate level (see Note K). Prior to 2017, the Company had six reporting units equivalent to its then six operating segments as follows: Bauxite, Alumina, Aluminum, Cast Products, Energy, and Rolled Products. In early 2017, management initiated a realignment of the Company’s internal business and organizational structure resulting in a change to Alcoa Corporation’s operating segments and reporting units (see Note E). Of the previous six reporting units, only Bauxite and Alumina contained goodwill. In reviewing goodwill for impairment, an entity has the option to first assess qualitative factors to determine whether the existence of events or circumstances leads to a determination that it is more likely than not (greater than 50%) that the estimated fair value of a reporting unit is less than its carrying amount. If an entity elects to perform a qualitative assessment and determines that an impairment is more likely than not, the entity is then required to perform the existing two-step two-step two-step Alcoa Corporation’s policy for its annual review of goodwill is to perform the qualitative assessment for all reporting units not subjected directly to the two-step two-step Under the qualitative assessment, various events and circumstances (or factors) that would affect the estimated fair value of a reporting unit are identified (similar to impairment indicators above). These factors are then classified by the type of impact they would have on the estimated fair value using positive, neutral, and adverse categories based on current business conditions. Additionally, an assessment of the level of impact that a particular factor would have on the estimated fair value is determined using high, medium, and low weighting. Furthermore, management considers the results of the most recent two-step During the 2017 annual review of goodwill, management performed the qualitative assessment for the Bauxite and Alumina reporting units. Management concluded it was not more likely than not that the respective estimated fair value of these reporting units was less than the respective carrying value. As such, no further analysis was required. Under the two-step In the event the estimated fair value of a reporting unit per the DCF model is less than the carrying value, additional analysis would be required. The additional analysis would compare the carrying amount of the reporting unit’s goodwill with the implied fair value of that goodwill, which may involve the use of valuation experts. The implied fair value of goodwill is the excess of the fair value of the reporting unit over the fair value amounts assigned to all of the assets and liabilities of that unit as if the reporting unit was acquired in a business combination and the fair value of the reporting unit represented the purchase price. If the carrying value of goodwill exceeds its implied fair value, an impairment loss equal to such excess would be recognized. Management last proceeded directly to the two-step Intangible assets with finite useful lives are amortized generally on a straight-line basis over the periods benefited. The following table details the weighted-average useful lives of software and other intangible assets by type of operation (numbers in years): Segment Software Other intangible assets Bauxite mining 9 10 Alumina refining 6 20 Aluminum smelting and casting 5 36 Energy generation 3 29 Aluminum rolling 4 20
Asset Retirement Obligations	Asset Retirement Obligations. Certain conditional asset retirement obligations (CAROs) related to alumina refineries, aluminum smelters, rolling mills, and energy generation facilities have not been recorded in the Consolidated Financial Statements due to uncertainties surrounding the ultimate settlement date. A CARO is a legal obligation to perform an asset retirement activity in which the timing and/or method of settlement are conditional on a future event that may or may not be within Alcoa Corporation’s control. Such uncertainties exist as a result of the perpetual nature of the structures, maintenance and upgrade programs, and other factors. At the date a reasonable estimate of the ultimate settlement date can be made (e.g., planned demolition), Alcoa Corporation would record an ARO for the removal, treatment, transportation, storage, and/or disposal of various regulated assets and hazardous materials such as asbestos, underground and aboveground storage tanks, polychlorinated biphenyls (PCBs), various process residuals, solid wastes, electronic equipment waste, and various other materials. Such amounts may be material to the Consolidated Financial Statements in the period in which they are recorded.
Environmental Matters	Environmental Matters.
Litigation Matters	Litigation Matters.
Revenue Recognition	Revenue Recognition.
Stock-Based Compensation	Stock-Based Compensation. Compensation expense for employee equity grants is recognized using the non-substantive Most plan participants can choose whether to receive their award in the form of stock options, stock awards, or a combination of both. This choice is made before the grant is issued and is irrevocable.
Pensions and Other Postretirement Benefit Plans	Pension and Other Postretirement Benefit Plans. non-Alcoa Prior to the Separation Date, certain other plans that were entirely attributable to employees of Alcoa Corporation-related operations (the “Direct Plans”) were accounted for as defined benefit pension and other postretirement benefit plans. Accordingly, the funded and unfunded position of each Direct Plan was recorded in the Consolidated Balance Sheet. Actuarial gains and losses that had not yet been recognized through earnings were recorded in accumulated other comprehensive income, net of taxes, until they were amortized as a component of net periodic benefit cost. The determination of benefit obligations and recognition of expenses related to the Direct Plans is dependent on various assumptions. The major assumptions primarily relate to discount rates, long-term expected rates of return on plan assets, and future compensation increases. ParentCo’s management developed each assumption using relevant company experience in conjunction with market-related data for each individual location in which such plans exist. In preparation for the Separation Transaction, effective August 1, 2016, certain of the Shared Plans were separated into standalone plans for both Alcoa Corporation and ParentCo (see Note N). Additionally, certain of the other remaining Shared Plans were assumed by Alcoa Corporation (See Note N). Accordingly, beginning on August 1, 2016 and forward, the standalone plans and assumed plans were accounted for as defined benefit pension and other postretirement plans. Additionally, the Direct Plans continued to be accounted for as defined benefit pension and other postretirement plans.
Derivatives and Hedging	Derivatives and Hedging. Alcoa Corporation accounts for hedges of firm customer commitments for aluminum as fair value hedges. As a result, the fair values of the derivatives and changes in the fair values of the underlying hedged items are reported as assets and liabilities in the Consolidated Balance Sheet. Changes in the fair values of these derivatives and underlying hedged items generally offset and are recorded each period in Sales, consistent with the underlying hedged item. Alcoa Corporation accounts for hedges of foreign currency exposures and certain forecasted transactions as cash flow hedges. The fair values of the derivatives are recorded as assets and liabilities in the Consolidated Balance Sheet. The effective portions of the changes in the fair values of these derivatives are recorded in Other comprehensive (loss) income and are reclassified to Sales, Cost of goods sold, or Other (income) expenses, net in the period in which earnings are impacted by the hedged items or in the period that the transaction no longer qualifies as a cash flow hedge. These contracts cover the same periods as known or expected exposures, generally not exceeding five years. If no hedging relationship is designated, the derivative is marked to market through Other (income) expenses, net. Cash flows from derivatives are recognized in the Statement of Consolidated Cash Flows in a manner consistent with the underlying transactions.
Income Taxes	Income Taxes. In all periods prior to the Separation Date, Alcoa Corporation’s operations were included in the income tax filings of ParentCo. The provision for income taxes in Alcoa Corporation’s Statement of Consolidated Operations was determined in the same manner described above, but on a separate return methodology as if the Company was a standalone taxpayer filing hypothetical income tax returns where applicable. Any additional accrued tax liability or refund arising as a result of this approach was assumed to be immediately settled with ParentCo as a component of Parent Company net investment. Deferred tax assets were also determined in the same manner described above and were reflected in the Consolidated Balance Sheet for net operating losses, credits or other attributes to the extent that such attributes were expected to transfer to Alcoa Corporation upon the Separation Transaction. Any difference from attributes generated in a hypothetical return on a separate return basis was adjusted as a component of Parent Company net investment. Valuation allowances are recorded to reduce deferred tax assets when it is more likely than not (greater than 50%) that a tax benefit will not be realized. In evaluating the need for a valuation allowance, management considers all potential sources of taxable income, including income available in carryback periods, future reversals of taxable temporary differences, projections of taxable income, and income from tax planning strategies, as well as all available positive and negative evidence. Positive evidence includes factors such as a history of profitable operations, projections of future profitability within the carryforward period, including from tax planning strategies, and Alcoa Corporation’s experience with similar operations. Existing favorable contracts and the ability to sell products into established markets are additional positive evidence. Negative evidence includes items such as cumulative losses, projections of future losses, or carryforward periods that are not long enough to allow for the utilization of a deferred tax asset based on existing projections of income. Deferred tax assets for which no valuation allowance is recorded may not be realized upon changes in facts and circumstances, resulting in a future charge to establish a valuation allowance. Existing valuation allowances are re-examined re-measured Tax benefits related to uncertain tax positions taken or expected to be taken on a tax return are recorded when such benefits meet a more likely than not threshold. Otherwise, these tax benefits are recorded when a tax position has been effectively settled, which means that the statute of limitation has expired or the appropriate taxing authority has completed their examination even though the statute of limitations remains open. Interest and penalties related to uncertain tax positions are recognized as part of the provision for income taxes and are accrued beginning in the period that such interest and penalties would be applicable under relevant tax law until such time that the related tax benefits are recognized.
Foreign Currency	Foreign Currency.
Recently Adopted Accounting Guidance	Recently Adopted Accounting Guidance. (last-in, first-out) non-LIFO On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to derivative instruments designated as hedging instruments. These changes clarify that a change in the counterparty to a derivative instrument that has been designated as a hedging instrument does not, in and of itself, require de-designation On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to equity method investments. These changes eliminate the requirement for an investor to adjust an equity method investment, results of operations, and retained earnings retroactively on a step-by-step available-for-sale On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to employee share-based payment accounting. Prior to these changes, an entity must determine for each share-based payment award whether the difference between the deduction for tax purposes and the compensation cost recognized for financial reporting purposes results in either an excess tax benefit or a tax deficiency. Excess tax benefits are recognized in additional paid-in tax-withholding tax-withholding On January 1, 2017, Alcoa Corporation adopted changes issued by the FASB to consolidation accounting. Prior to these changes, an entity was required to consider indirect economic interests in a variable interest entity held through related parties under common control as direct interests in their entirety in the entity’s assessment of whether it is the primary beneficiary of the variable interest entity. The changes result in an entity considering such indirect economic interests only on a proportionate basis as indirect interests instead of as direct interests in their entirety. The adoption of these changes had no impact on the Consolidated Financial Statements; however, this guidance will need to be considered in future assessments of whether Alcoa Corporation is the primary beneficiary of a variable interest entity.
Recently Issued Accounting Guidance	Recently Issued Accounting Guidance. In January 2017, the FASB issued changes to the assessment of goodwill for impairment as it relates to the quantitative test. Currently, there are two steps when performing a quantitative impairment test. The first step requires an entity to compare the current fair value of a reporting unit to its carrying value. In the event the reporting unit’s estimated fair value is less than its carrying value, an entity performs the second step, which is to compare the carrying amount of the reporting unit’s goodwill with the implied fair value of that goodwill. The implied fair value of goodwill is the excess of the fair value of the reporting unit over the fair value amounts assigned to all of the assets and liabilities of that unit as if the reporting unit was acquired in a business combination and the fair value of the reporting unit represented the purchase price. If the carrying value of goodwill exceeds its implied fair value, an impairment loss equal to such excess would be recognized. These changes eliminate the second step of the quantitative impairment test. Accordingly, an entity would recognize an impairment of goodwill for a reporting unit, if under what is currently referred to as the first step, the estimated fair value of the reporting unit is less than the carrying value. The impairment would be equal to the excess of the reporting unit’s carrying value over its fair value not to exceed the total amount of goodwill applicable to that reporting unit. These changes become effective for Alcoa Corporation on January 1, 2020. Management has determined that the adoption of these changes will not have an immediate impact on the Consolidated Financial Statements. This guidance will need to be considered each time Alcoa Corporation performs an assessment of goodwill for impairment under the quantitative test. In March 2017, the FASB issued changes to the presentation of net periodic benefit cost related to pension and other postretirement benefit plans. These changes require that an entity report the service cost component of net periodic benefit cost in the same line item(s) on the statement of operations as other compensation costs arising from services rendered by the pertinent employees during a reporting period. The other components of net periodic benefit cost (see Note N) are required to be presented separately from the service cost component. In other words, these other components may be aggregated and presented as a separate line item or they may be included in existing line items on the statement of operations other than such line items that include the service cost component. Currently, Alcoa Corporation includes all components of net periodic benefit cost, except for certain settlements, curtailments, and special termination benefits related to severance programs, in Cost of goods sold (business employees) and Selling, general administrative, and other expenses (corporate employees) consistent with the location of other compensation costs related to the respective employees. The non-service non-service non-service In May 2017, the FASB issued changes to the accounting for stock-based compensation when there has been a modification to the terms or conditions of a share-based payment award. These changes require an entity to account for the modification only when there has been a substantive change to the terms or conditions of a share-based payment award. A substantive change occurs when the fair value, vesting conditions or balance sheet classification (liability or equity) of a share-based payment award is/are different immediately before and after the modification. Currently, an entity is required to account for any modification in the terms or conditions of a share-based payment award. These changes become effective for Alcoa Corporation on January 1, 2018. Management has determined that the adoption of these changes will not have an immediate impact on the Consolidated Financial Statements. Additionally, the Company will no longer account for any future non-substantive In August 2017, the FASB issued changes to the accounting for hedging activities. These changes permit hedge accounting for risk components in hedging relationships involving nonfinancial risk and interest rate risk; reduce current limitations on the designation and measurement of a hedged item in a fair value hedge of interest rate risk; remove the requirement to separately measure and report hedge ineffectiveness; provide an election to systematically and rationally recognize in earnings the initial value of any amount excluded from the assessment of hedge effectiveness for all types of hedges; and ease the requirements of effectiveness testing. Additionally, modifications to existing disclosures, as well as additional disclosures, will be required to reflect these changes regarding the measurement and recording of hedging activities. These changes become effective for Alcoa Corporation on January 1, 2019 (early adoption is permitted). Management is currently evaluating early adopting these changes and the potential impact of these changes on the Consolidated Financial Statements. In January 2016, the FASB issued changes to the accounting and reporting of certain equity investments. These changes require equity investments (except those accounted for under the equity method of accounting or those that result in consolidation of the investee) to be measured at fair value with changes in fair value recognized in net income. However, an entity may choose to measure equity investments that do not have readily determinable fair values at cost minus impairment, if any, plus or minus changes resulting from observable price changes in orderly transactions for the identical or similar investment of the same issuer. Additionally, the impairment assessment of equity investments without readily determinable fair values has been simplified by requiring a qualitative assessment to identify impairment. These changes become effective for Alcoa Corporation on January 1, 2018. Management has determined that the adoption of these changes will not have an impact on the Consolidated Financial Statements, as all of Alcoa Corporation’s equity investments are accounted for under the equity method of accounting. In February 2016, the FASB issued changes to the accounting and presentation of leases. These changes require lessees to recognize a right of use asset and lease liability on the balance sheet for all leases with terms longer than 12 months. For leases with a term of 12 months or less, a lessee is permitted to make an accounting policy election by class of underlying asset not to recognize a right of use asset and lease liability. Additionally, when measuring assets and liabilities arising from a lease, optional payments should be included only if the lessee is reasonably certain to exercise an option to extend the lease, exercise a purchase option, or not exercise an option to terminate the lease. These changes become effective for Alcoa Corporation on January 1, 2019. The Company has established a cross-functional project team to lead the implementation effort. This team has determined that the Company requires information systems updates and incremental software to effectively implement these changes. Accordingly, the Company has selected a software vendor and is in the early stages of implementing lease management software. Upon adoption of these changes, management does expect to record a right of use asset and lease liability on Alcoa Corporation’s Consolidated Balance Sheet. While the precise amount of this asset and liability will not be known until closer to the adoption date, management estimates the amount to be less than 5% of both total assets and total liabilities. This estimate is based on Alcoa Corporation’s Consolidated Balance Sheet and lease portfolio, both as of December 31, 2017. In June 2016, the FASB added a new impairment model (known as the current expected credit loss (CECL) model) that is based on expected losses rather than incurred losses. Under the new guidance, an entity recognizes as an allowance its estimate of expected credit losses. The CECL model applies to most debt instruments, trade receivables, lease receivables, financial guarantee contracts, and other loan commitments. The CECL model does not have a minimum threshold for recognition of impairment losses and entities will need to measure expected credit losses on assets that have a low risk of loss. These changes become effective for Alcoa Corporation on January 1, 2020. Management is currently evaluating the potential impact of these changes on the Consolidated Financial Statements. In both August and November 2016, the FASB issued changes to the presentation of a number of items in the statement of cash flows. Specifically, the changes identify nine specific cash flow items and the sections where they must be presented within the statement of cash flows, including distributions received from equity method investees, proceeds from the settlement of insurance claims, and restricted cash. These changes become effective for Alcoa Corporation on January 1, 2018. Management has determined that the adoption of these changes will not have a material impact on the Consolidated Financial Statements. In October 2016, the FASB issued changes to the accounting for intra-entity transactions, other than inventory. Currently, no immediate tax impact is recognized in an entity’s financial statements as a result of intra-entity transfers of assets. An entity is precluded from reflecting a tax benefit or expense from an intra-entity asset transfer between entities that file separate tax returns, whether or not such entities are in different tax jurisdictions, until the asset has been sold to a third party or otherwise recovered. The buyer of such asset is prohibited from recognizing a deferred tax asset for the temporary difference arising from the excess of the buyer’s tax basis over the cost to the seller. The changes require the current and deferred income tax consequences of the intra-entity transfer to be recorded when the transaction occurs. The exception to defer the tax consequences of inventory transactions is maintained. These changes become effective for Alcoa Corporation on January 1, 2018. Management has determined that the adoption of these changes will not have a material impact on the Consolidated Financial Statements. In May 2014, the FASB issued changes to the recognition of revenue from contracts with customers. These changes created a comprehensive framework for all entities in all industries to apply in the determination of when to recognize revenue, and, therefore, supersede virtually all existing revenue recognition requirements and guidance. This framework is expected to result in less complex guidance in application while providing a consistent and comparable methodology for revenue recognition. The core principle of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. To achieve this principle, an entity should apply the following steps: (i) identify the contract(s) with a customer, (ii) identify the performance obligations in the contract(s), (iii) determine the transaction price, (iv) allocate the transaction price to the performance obligations in the contract(s), and (v) recognize revenue when, or as, the entity satisfies a performance obligation. In August 2015, the FASB deferred the effective date by one year, making these changes effective for Alcoa Corporation on January 1, 2018. Through a previously established project team, the Company completed a detailed review of the terms and provisions of its customer contracts by mid-2017.

Basis of Presentation (Tables)

Basis of Presentation (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Schedule of Cost Allocation	The following table reflects the allocations described above: 2016 2015 Cost of goods sold (1) $ 40 $ 93 Selling, general administrative, and other expenses (2) 150 146 Research and development expenses 2 17 Provision for depreciation, depletion, and amortization 18 22 Restructuring and other charges (3) 1 32 Interest expense 198 245 Other (income) expenses, net $ (7 ) $ 12 (1) Allocation principally relates to expenses for ParentCo’s retained pension and other postretirement benefits associated with closed and sold operations. (2) Allocation includes costs incurred by ParentCo associated with the Separation Transaction (see Separation Transaction above). (3) Allocation primarily relates to layoff programs for ParentCo corporate employees.

Summary of Significant Accoun29

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2017
Accounting Policies [Abstract]
Weighted-Average Useful Lives of Structures and Machinery and Equipment	The following table details the weighted-average useful lives of structures and machinery and equipment by type of operation (numbers in years): Segment Structures Machinery and equipment Bauxite mining 35 17 Alumina refining 30 28 Aluminum smelting and casting 36 22 Energy generation 33 25 Aluminum rolling 31 17
Weighted-Average Useful Lives of Software and Other Intangible Assets	The following table details the weighted-average useful lives of software and other intangible assets by type of operation (numbers in years): Segment Software Other intangible assets Bauxite mining 9 10 Alumina refining 6 20 Aluminum smelting and casting 5 36 Energy generation 3 29 Aluminum rolling 4 20

Restructuring and Other Charg30

Restructuring and Other Charges (Tables)	12 Months Ended
	Dec. 31, 2017
Restructuring and Related Activities [Abstract]
Schedule of Restructuring and Other Charges	Restructuring and other charges for each year in the three-year period ended December 31, 2017 were comprised of the following: 2017 2016 2015 Early termination of a power contract $ 244 $ - $ - Asset impairments 40 155 311 Layoff costs 23 32 199 Legal matters in Italy (R) (22 ) - 201 Asset retirement obligations (Q) 10 97 76 Environmental remediation (R) 8 26 86 Net (gain) loss on divestitures of businesses (C) - (3 ) 25 Other 49 47 92 Reversals of previously recorded layoff and other costs (43 ) (36 ) (7 ) Restructuring and other charges $ 309 $ 318 $ 983 * In 2016 and 2015, Other includes $1 and $32, respectively, related to the allocation of restructuring charges to Alcoa Corporation from ParentCo (see Note A).
Schedule of Restructuring and Other Charges by Reportable Segments, Pretax	Alcoa Corporation does not include Restructuring and other charges in the results of its reportable segments. The impact of allocating such charges to segment results would have been as follows: 2017 2016 2015 Bauxite $ 2 $ (5 ) $ 8 Alumina 3 72 102 Aluminum 51 75 100 Segment total 56 142 210 Corporate 253 176 773 Total restructuring and other charges $ 309 $ 318 $ 983
Activity and Reserve Balances for Restructuring Charges	Activity and reserve balances for restructuring charges were as follows: Layoff Other Total Reserve balances at December 31, 2014 $ 50 $ 13 $ 63 2015: Cash payments (65 ) (1 ) (66 ) Restructuring charges 199 222 421 Other* (47 ) (219 ) (266 ) Reserve balances at December 31, 2015 137 15 152 2016: Cash payments (74 ) (35 ) (109 ) Restructuring charges 32 168 200 Other* (57 ) (120 ) (177 ) Reserve balances at December 31, 2016 38 28 66 2017: Cash payments (30 ) (43 ) (73 ) Restructuring charges 23 67 90 Other* (20 ) (18 ) (38 ) Reserve balances at December 31, 2017 $ 11 $ 34 $ 45 * Other includes reversals of previously recorded restructuring charges and the effects of foreign currency translation. In 2017, 2016, and 2015, Other for Layoff costs also included a reclassification of $8, $16, and $35, respectively, in pension and/or other postretirement benefits costs, as these obligations were included in Alcoa Corporation’s separate liability for pension and other postretirement benefits obligations (see Note N). Additionally in 2017, 2016, and 2015, Other for Other costs also included a reclassification of the following restructuring charges: $10, $97, and $76, respectively, in asset retirement and $8, $26, and $86, respectively, in environmental obligations, as these liabilities were included in Alcoa Corporation’s separate reserves for asset retirement obligations (see Note Q) and environmental remediation (see Note R).

Segment and Geographic Area I31

Segment and Geographic Area Information (Tables)	12 Months Ended
	Dec. 31, 2017
Segment Reporting [Abstract]
Schedule of Operating Results,Capital Expenditures and Assets of Alcoa's Reportable Segments	The operating results, capital expenditures, and assets of Alcoa Corporation’s reportable segments were as follows: Bauxite Alumina Aluminum Total 2017 Sales: Third-party sales—unrelated party $ 333 $ 3,133 $ 7,163 $ 10,629 Third-party sales—related party - - 864 864 Intersegment sales 875 1,723 21 2,619 Total sales $ 1,208 $ 4,856 $ 8,048 $ 14,112 Adjusted EBITDA $ 427 $ 1,289 $ 964 $ 2,680 Supplemental information: Depreciation, depletion, and amortization $ 82 $ 207 $ 419 $ 708 Equity loss - (5 ) (19 ) (24 ) 2016 Sales: Third-party sales—unrelated party $ 315 $ 2,300 $ 5,573 $ 8,188 Third-party sales—related party - - 958 958 Intersegment sales 751 1,307 42 2,100 Total sales $ 1,066 $ 3,607 $ 6,573 $ 11,246 Adjusted EBITDA $ 375 $ 378 $ 680 $ 1,433 Supplemental information: Depreciation, depletion, and amortization $ 77 $ 186 $ 414 $ 677 Equity loss - (40 ) (24 ) (64 ) 2015 Sales: Third-party sales—unrelated party $ 71 $ 3,341 $ 6,479 $ 9,891 Third-party sales—related party - - 1,078 1,078 Intersegment sales 1,076 1,727 175 2,978 Total sales $ 1,147 $ 5,068 $ 7,732 $ 13,947 Adjusted EBITDA $ 454 $ 958 $ 767 $ 2,179 Supplemental information: Depreciation, depletion, and amortization $ 93 $ 192 $ 424 $ 709 Equity loss - (41 ) (44 ) (85 ) 2017 Assets: Capital expenditures $ 53 $ 144 $ 178 $ 375 Equity investments 191 262 930 1,383 Total assets 1,609 5,129 8,060 14,798 2016 Assets: Capital expenditures $ 29 $ 109 $ 256 $ 394 Equity investments 163 342 859 1,364 Total assets 1,541 4,791 7,658 13,990
Schedule of Reconciliation of Certain Segment Information to Consolidated Totals	The following tables reconcile certain segment information to consolidated totals: 2017 2016 2015 Sales: Total segment sales $ 14,112 $ 11,246 $ 13,947 Elimination of intersegment sales (2,619 ) (2,100 ) (2,978 ) Other 159 172 230 Consolidated sales $ 11,652 $ 9,318 $ 11,199
Schedule of Segment ATOI to Combined Net Income (Loss) Attributable to Alcoa Corporation	2017 2016 2015 Net income (loss) attributable to Alcoa Corporation: Total segment Adjusted EBITDA $ 2,680 $ 1,433 $ 2,179 Unallocated amounts: Impact of LIFO (I) (91 ) (10 ) 107 Metal price lag (1) 26 9 (30 ) Corporate expense (2) (136 ) (177 ) (173 ) Provision for depreciation, depletion, and amortization (750 ) (718 ) (780 ) Restructuring and other charges (D) (309 ) (318 ) (983 ) Interest expense (S) (104 ) (243 ) (270 ) Other income (expenses), net (S) 58 89 (42 ) Other (3) (215 ) (227 ) (345 ) Consolidated income (loss) before income taxes 1,159 (162 ) (337 ) Provision for income taxes (P) (600 ) (184 ) (402 ) Net income attributable to noncontrolling interest (342 ) (54 ) (124 ) Consolidated net income (loss) attributable to Alcoa Corporation $ 217 $ (400 ) $ (863 ) (1) Metal price lag describes the timing difference created when the average price of metal sold differs from the average cost of the metal when purchased by Alcoa Corporation’s rolled aluminum operations. In general, when the price of metal increases, metal price lag is favorable, and when the price of metal decreases, metal price lag is unfavorable. (2) Corporate expense is primarily composed of general administrative and other expenses of operating the corporate headquarters and other global administrative facilities. (3) Other includes, among other items, the Adjusted EBITDA of previously closed operations as applicable, pension and other postretirement benefit expenses associated with closed and sold operations, and intersegment profit elimination.
Schedule of Segment Reporting Information to Consolidated Assets	December 31, 2017 2016 Assets: Total segment assets $ 14,798 $ 13,990 Elimination of intersegment receivables (299 ) (236 ) Unallocated amounts: Cash and cash equivalents 1,358 853 LIFO reserve (306 ) (215 ) Corporate fixed assets, net 520 595 Corporate goodwill 148 149 Deferred income taxes 814 741 Fair value of derivative contracts 34 497 Other 380 367 Consolidated assets $ 17,447 $ 16,741
Schedule of Sales By Major Product Grouping	Sales by major product grouping were as follows: 2017 2016 2015 Sales: Primary aluminum $ 6,168 $ 5,204 $ 6,214 Alumina 3,121 2,280 3,325 Flat-rolled aluminum 1,666 1,068 989 Energy 446 422 588 Bauxite 333 315 71 Other* (82 ) 29 12 $ 11,652 $ 9,318 $ 11,199 * Other includes realized gains and losses related to embedded derivative instruments designated as cash flow hedges of forward sales of aluminum (see Note O).
Schedule of Geographic Information for Sales	Geographic information for sales was as follows (based upon the country where the point of sale originated): 2017 2016 2015 Sales: United States (1) $ 5,370 $ 4,365 $ 5,386 Spain (2) 3,303 2,663 2,852 Australia 2,266 1,644 2,147 Brazil 569 432 562 Canada 93 141 132 Other 51 73 120 $11,652 $9,318 $11,199 (1) Sales of a portion of the alumina from refineries in Australia, Brazil, and Suriname (prior to closure in December 2016) and most of the aluminum from smelters in Canada occurred in the United States. (2) Sales of the aluminum produced from smelters in Iceland and Norway, as well as the off-take
Schedule of Geographic Information for Long-Lived Assets	Geographic information for long-lived assets was as follows (based upon the physical location of the assets): December 31, 2017 2016 Long-lived assets: Australia $ 2,220 $ 2,053 Brazil 2,111 2,228 United States 1,658 1,816 Iceland 1,276 1,341 Canada 1,116 1,161 Norway 427 438 Spain 316 273 Other 14 15 $9,138 $9,325

Earnings Per Share (Tables)

Earnings Per Share (Tables)	12 Months Ended
	Dec. 31, 2017
Earnings Per Share [Abstract]
Schedule of Computation of Basic and Diluted EPS Attributable to Alcoa Corporation Common Shareholders	The information used to compute basic and diluted EPS attributable to Alcoa Corporation common shareholders was as follows (shares in millions): 2017 2016 2015 Net income (loss) attributable to Alcoa Corporation $ 217 $ (400 ) $ (863 ) Average shares outstanding—basic 184 183 182 Effect of dilutive securities: Stock options 1 - - Stock and performance awards 2 - - Average shares outstanding—diluted 187 183 182

Accumulated Other Comprehensi33

Accumulated Other Comprehensive Loss (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component	The following table details the activity of the three components that comprise Accumulated other comprehensive loss for both Alcoa Corporation’s shareholders and noncontrolling interest: Alcoa Corporation Noncontrolling interest 2017 2016 2015 2017 2016 2015 Pension and other postretirement benefits (N) Balance at beginning of period $ (2,330 ) $ (352 ) $ (424 ) $ (56 ) $ (56 ) $ (64 ) Establishment of additional defined benefit plans - (2,704 ) - - - - Separation-related adjustments (A) - 928 - - - - Other comprehensive (loss) income: Unrecognized net actuarial loss and prior service cost/benefit (671 ) (307 ) 73 9 2 5 Tax benefit (expense) 25 6 (18 ) (2 ) (6 ) (1 ) Total Other comprehensive (loss) income before reclassifications, net of tax (646 ) (301 ) 55 7 (4 ) 4 Amortization of net actuarial loss and prior service cost/benefit (1) 199 107 26 2 5 6 Tax expense (2) (9 ) (8 ) (9 ) - (1 ) (2 ) Total amount reclassified from Accumulated other comprehensive loss, net of tax (7) 190 99 17 2 4 4 Total Other comprehensive (loss) income (456 ) (202 ) 72 9 - 8 Balance at end of period $ (2,786 ) $ (2,330 ) $ (352 ) $ (47 ) $ (56 ) $ (56 ) Foreign currency translation Balance at beginning of period $ (1,655 ) $ (1,851 ) $ (668 ) $ (677 ) $ (779 ) $ (351 ) Separation-related adjustments (A) - (17 ) - - - - Other comprehensive income (loss) (3) 188 213 (1,183 ) 96 102 (428 ) Balance at end of period $ (1,467 ) $ (1,655 ) $ (1,851 ) $ (581 ) $ (677 ) $ (779 ) Cash flow hedges (O) Balance at beginning of period $ 210 $ 603 $ (224 ) $ 1 $ (3 ) $ (2 ) Separation-related adjustments (A) - (47 ) - - - - Other comprehensive (loss) income: Net change from periodic revaluations (1,489 ) (558 ) 1,155 83 38 (1 ) Tax benefit (expense) 251 233 (344 ) (25 ) (12 ) - Total Other comprehensive (loss) income before reclassifications, net of tax (1,238 ) (325 ) 811 58 26 (1 ) Net amount reclassified to earnings: Aluminum contracts (4) 130 7 21 - - - Financial contract (5) (19 ) (54 ) - (12 ) (37 ) - Foreign exchange contracts (4) (2 ) - - - - - Interest rate contract (6) - 7 - - 5 - Sub-total 109 (40 ) 21 (12 ) (32 ) - Tax (expense) benefit (2) (10 ) 19 (5 ) 4 10 - Total amount reclassified from Accumulated other comprehensive loss, net of tax (7) 99 (21 ) 16 (8 ) (22 ) - Total Other comprehensive (loss) income (1,139 ) (346 ) 827 50 4 (1 ) Balance at end of period $ (929 ) $ 210 $ 603 $ 51 $ 1 $ (3 ) (1) These amounts were included in the computation of net periodic benefit cost for pension and other postretirement benefits (see Note N). (2) These amounts were included in Provision for income taxes on the accompanying Statement of Consolidated Operations. (3) In all periods presented, there were no tax impacts related to rate changes and no amounts were reclassified to earnings. (4) These amounts were included in Sales on the accompanying Statement of Consolidated Operations. (5) The 2017 amounts were included in Cost of goods sold on the accompanying Statement of Consolidated Operations. The 2016 and 2015 amounts were included in Other (income) expenses, net on the accompanying Statement of Consolidated Operations. (6) These amounts were included in Other (income) expenses, net on the accompanying Statement of Consolidated Operations. (7) A positive amount indicates a corresponding charge to earnings and a negative amount indicates a corresponding benefit to earnings. These amounts were reflected on the accompanying Statement of Consolidated Operations in the line items indicated in footnotes 1 through 6.

Investments (Tables)

Investments (Tables)	12 Months Ended
	Dec. 31, 2017
Equity Method Investments and Joint Ventures [Abstract]
Summary of Investment	December 31, 2017 2016 Equity investments $ 1,400 $ 1,348 Other investments 10 10 $1,410 $1,358
Schedule of Equity Investment	The following table summarizes information of the equity investments as of December 31, 2017 and 2016 (AofA sold its interest in the Dampier to Bunbury Natural Gas Pipeline (DBNGP) Trust in April 2016 (see DBNGP Trust below)): Investee Country Nature of investment (4) Ownership interest Ma’aden Aluminum Company (1) Saudi Arabia Aluminum smelter 25.1 % Ma’aden Bauxite and Alumina Company (1) Saudi Arabia Bauxite mine and alumina refinery 25.1 % (5) Ma’aden Rolling Company (1) Saudi Arabia Aluminum rolling mill 25.1 % Halco Mining, Inc. (2) Guinea Bauxite mine 45 % (5) Energetica Barra Grande S.A. Brazil Hydroelectric generation facility 42.18 % Pechiney Reynolds Quebec, Inc. (3) Canada Aluminum smelter 50 % Consorcio Serra do Facão Brazil Hydroelectric generation facility 34.97 % Mineração Rio do Norte S.A. Brazil Bauxite mine 18.2 % (5) Manicouagan Power Limited Partnership Canada Hydroelectric generation facility 40 % (1) See Saudi Arabia Joint Venture below for additional information. (2) Halco Mining, Inc. owns 100% of Boké Investment Company, which owns 51% of Compagnie des Bauxites de Guinée. (3) Pechiney Reynolds Quebec, Inc. owns a 50.1% interest in the Bécancour smelter in Quebec, Canada thereby entitling Alcoa Corporation to a 25.05% interest in the smelter. Through two wholly-owned Canadian subsidiaries, Alcoa Corporation also owns 49.9% of the Bécancour smelter. (4) Each of the investees either owns the facility listed or has an ownership interest in an entity that owns the facility listed. (5) A portion or all of each of these ownership interests are held by majority-owned subsidiaries that are part of AWAC.
Summary of Financial Information for Alcoa Corporation's Equity Investments	Financial information for these equity investments is as follows (amounts represent 100% of the investee’s financial information): Saudi Arabia (1) Halco Energetica Pechiney Consorcio Mineração Manicouagan Power L.P. DBNGP (2) Total Profit and loss data—year ended December 31, 2017 Sales $ 3,032 $ 416 $ 131 $ 332 $ 103 $ 350 $ 105 $ - $ 4,469 Cost of goods sold 2,776 266 117 292 48 273 9 - 3,781 (Loss) Income before income taxes (142 ) 50 13 35 45 35 96 - 132 Net (loss) income (157 ) 47 5 23 46 30 88 - 82 Equity in net (loss) income of affiliated companies, before reconciling adjustments (39 ) 21 2 11 16 6 35 - 52 Other 9 (1 ) - - - - 2 - 10 Alcoa Corporation’s equity in net (loss) income of affiliated companies (30 ) 20 2 11 16 6 37 - 62 Profit and loss data—year ended December 31, 2016 Sales $ 1,970 $ 437 $ 60 $ 309 $ 90 $ 451 $ 104 $ 86 $ 3,507 Cost of goods sold 1,905 242 35 272 65 297 10 18 2,844 (Loss) Income before income taxes (295 ) 53 16 36 8 152 94 21 85 Net (loss) income (295 ) 50 15 16 5 129 87 14 21 Equity in net (loss) income of affiliated companies, before reconciling adjustments (75 ) 23 6 8 2 23 35 3 25 Other 7 2 (1 ) (4 ) - (1 ) - - 3 Alcoa Corporation’s equity in net (loss) income of affiliated companies (68 ) 25 5 4 2 22 35 3 28 Profit and loss data—year ended December 31, 2015 Sales $ 2,025 $ 487 $ 130 $ 486 $ 84 $ 498 $ 106 $ 303 $ 4,119 Cost of goods sold 2,070 236 98 288 76 308 16 117 3,209 (Loss) Income before income taxes (362 ) 86 27 113 6 118 91 46 125 Net (loss) income (366 ) 80 7 104 (1 ) 98 90 31 43 Equity in net (loss) income of affiliated companies, before reconciling adjustments (91 ) 36 3 52 - 18 36 6 60 Other (2 ) (2 ) (1 ) 4 (2 ) (3 ) - (1 ) (7 ) Alcoa Corporation’s equity in net (loss) income of affiliated companies (93 ) 34 2 56 (2 ) 15 36 5 53 Balance sheet data—as of December 31, 2017 Current assets $ 1,415 $ 40 $ 44 $ 140 $ 79 $ 121 $ 23 $ - $ 1,862 Noncurrent assets 9,373 174 285 106 261 744 72 - 11,015 Current liabilities 1,331 1 48 65 25 220 9 - 1,699 Noncurrent liabilities 6,191 12 6 12 117 413 - - 6,751 Balance Sheet data—as of December 31, 2016 Current assets $ 1,141 $ 12 $ 17 $ 91 $ 19 $ 92 $ 24 $ - $ 1,396 Noncurrent assets 9,653 189 302 147 451 644 68 - 11,454 Current liabilities 1,452 4 31 62 37 174 7 - 1,767 Noncurrent liabilities 6,204 14 23 - 283 253 - - 6,777 (1) The amounts included in this column represent the combined financial information related to Ma’aden Aluminum Company, Ma’aden Bauxite and Alumina Company, and Ma’aden Rolling Company. (2) AofA sold its interest in the DBNGP Trust in April 2016.

Inventories (Tables)

Inventories (Tables)	12 Months Ended
	Dec. 31, 2017
Inventory Disclosure [Abstract]
Schedule of Inventory Components	December 31, 2017 2016 Finished goods $ 296 $ 226 Work-in-process 258 220 Bauxite and alumina 585 429 Purchased raw materials 473 363 Operating supplies 147 137 LIFO reserve (306 ) (215 ) $ 1,453 $ 1,160

Properties, Plants, and Equip36

Properties, Plants, and Equipment, Net (Tables)	12 Months Ended
	Dec. 31, 2017
Property, Plant and Equipment [Abstract]
Schedule of Properties, Plants, and Equipment, Net	December 31, 2017 2016 Land and land rights, including mines $ 346 $ 346 Structures (by type of operation): Bauxite mining 1,250 1,194 Alumina refining 2,664 2,500 Aluminum smelting and casting 3,575 3,544 Energy generation 552 610 Aluminum rolling 298 287 Other 417 405 8,756 8,540 Machinery and equipment (by type of operation): Bauxite mining 508 465 Alumina refining 4,009 3,773 Aluminum smelting and casting 6,827 6,655 Energy generation 905 1,080 Aluminum rolling 1,020 884 Other 288 309 13,557 13,166 22,659 22,052 Less: accumulated depreciation, depletion, and amortization 13,908 13,225 8,751 8,827 Construction work-in-progress 387 498 $ 9,138 $ 9,325

Goodwill and Other Intangible37

Goodwill and Other Intangible Assets (Tables)	12 Months Ended
	Dec. 31, 2017
Goodwill and Intangible Assets Disclosure [Abstract]
Summary of Goodwill which is Included in Other Noncurret Assets	Goodwill, which is included in Other noncurrent assets on the accompanying Consolidated Balance Sheet, was as follows: December 31, 2017 2016 Bauxite $ 2 $ 2 Alumina 4 4 Aluminum (1) - - Corporate (2) 148 149 $ 154 $ 155 (1) The carrying value of Aluminum’s goodwill is zero, comprised of goodwill of $989 and accumulated impairment losses of $989 as of both December 31, 2017 and 2016. Additionally, the carrying value of Corporate’s goodwill is net of accumulated impairment losses of $742 as of both December 31, 2017 and 2016. (2) As of December 31, 2017, the $148 of goodwill reflected in Corporate is allocated to two of Alcoa Corporation’s three reportable segments ($49 to Bauxite and $99 to Alumina) for purposes of impairment testing (see Note B). This goodwill is reflected in Corporate for segment reporting purposes because it is not included in management’s assessment of performance by the two reportable segments.
Other Intangible Assets	Other intangible assets, which are included in Other noncurrent assets on the accompanying Consolidated Balance Sheet, were as follows: 2017 2016 December 31, Gross carrying amount Accumulated amortization Gross carrying amount Accumulated Computer software $ 241 $ (212 ) $ 252 $ (196 ) Patents and licenses* 25 (6 ) 70 (6 ) Other intangibles 21 (7 ) 21 (6 ) Total other intangible assets $ 287 $ (225 ) $ 343 $ (208 ) * As of December 31, 2016, Patents and licenses include amounts related to the capitalization of costs associated with the renewal of Alcoa Corporation’s FERC (Federal Energy Regulatory Commission) license at its Yadkin Hydroelectric Project, which was divested in February 2017 (see Note C).

Debt (Tables)

Debt (Tables)	12 Months Ended
	Dec. 31, 2017
Debt Disclosure [Abstract]
Schedule of Long-Term Debt	Long-Term Debt. December 31, 2017 2016 6.75% Notes, due 2024 $ 750 $ 750 7.00% Notes, due 2026 500 500 BNDES Loans, due 2018-2029 (see below for weighted average rates) 137 192 Other 50 38 Unamortized discounts and deferred financing costs (33 ) (35 ) 1,404 1,445 Less: amount due within one year 16 21 $ 1,388 $ 1,424 The principal amount of long-term debt maturing in each of the next five years is $16 in 2018, $30 in 2019, and $15 in each of 2020, 2021, and 2022.

Preferred and Common Stock (Tab

Preferred and Common Stock (Tables)	12 Months Ended
	Dec. 31, 2017
Equity [Abstract]
Schedule of Activity for Stock Options and Stock Awards	The activity for stock options and stock awards during 2017 was as follows: Stock options Stock awards Number of options Weighted average exercise price Number of awards Weighted average FMV per award Outstanding, January 1, 2017 4,108,535 $ 24.69 2,557,842 $ 22.65 Granted 331,681 37.68 796,330 37.39 Exercised (1,679,148 ) 25.80 - - Converted - - (764,064 ) 24.98 Expired or forfeited (73,961 ) 28.99 (145,756 ) 24.90 Performance share adjustment - - (143,020 ) 21.03 Outstanding, December 31, 2017 2,687,107 25.48 2,301,332 26.93

Pension and Other Postretirem40

Pension and Other Postretirement Benefits (Tables)	12 Months Ended
	Dec. 31, 2017
Schedule of Obligations and Funded Status	Obligations and Funded Status Pension benefits Other December 31, 2017 2016 2017 2016 Change in benefit obligation Benefit obligation at beginning of year $ 7,269 $ 2,246 $ 1,286 $ 82 Benefit obligation assumed on August 1, 2016 - 5,316 - 1,277 Service cost 84 74 5 2 Interest cost 250 142 38 16 Amendments 2 1 - - Actuarial losses (gains) 388 (244 ) (1 ) (33 ) Settlements (64 ) (80 ) - - Benefits paid, net of participants’ contributions (437 ) (218 ) (116 ) (61 ) Medicare Part D subsidy receipts - - 5 3 Foreign currency translation impact 147 32 1 - Benefit obligation at end of year* $ 7,639 $ 7,269 $ 1,218 $ 1,286 Change in plan assets Fair value of plan assets at beginning of year $ 5,421 $ 1,891 $ - $ - Fair value of plan assets assumed on August 1, 2016 - 4,065 - - Actual return on plan assets 187 (332 ) - - Employer contributions 111 72 - - Participant contributions 15 18 - - Benefits paid (432 ) (224 ) - - Administrative expenses (41 ) (11 ) - - Settlements (62 ) (80 ) - - Foreign currency translation impact 123 22 - - Fair value of plan assets at end of year* $ 5,322 $ 5,421 $ - $ - Funded status* $ (2,317 ) $ (1,848 ) $ (1,218 ) $ (1,286 ) Less: Amounts attributed to joint venture partners (37 ) (30 ) - - Net funded status $ (2,280 ) $ (1,818 ) $ (1,218 ) $ (1,286 ) Amounts recognized in the Consolidated Balance Sheet consist of: Noncurrent assets $ 72 $ 43 $ - $ - Current liabilities (11 ) (10 ) (118 ) (120 ) Noncurrent liabilities (2,341 ) (1,851 ) (1,100 ) (1,166 ) Net amount recognized $ (2,280 ) $ (1,818 ) $ (1,218 ) $ (1,286 ) Amounts recognized in Accumulated Other Comprehensive Loss consist of: Net actuarial loss $ 3,743 $ 3,254 $ 281 $ 295 Prior service cost (benefit) 35 42 (30 ) (36 ) Total, before tax effect 3,778 3,296 251 259 Less: Amounts attributed to joint venture partners 45 36 - - Net amount recognized, before tax effect $ 3,733 $ 3,260 $ 251 $ 259 Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Loss consist of: Net actuarial loss (benefit) $ 676 $ 337 $ (1 ) $ (33 ) Amortization of accumulated net actuarial loss (187 ) (105 ) (13 ) (8 ) Prior service cost (benefit) 2 2 - (1 ) Amortization of prior service (cost) benefit (9 ) (7 ) 6 5 Total, before tax effect 482 227 (8 ) (37 ) Less: Amounts attributed to joint venture partners 9 (3 ) - - Net amount recognized, before tax effect $ 473 $ 230 $ (8 ) $ (37 ) * At December 31, 2017, the benefit obligation, fair value of plan assets, and funded status for U.S. pension plans were $5,093, $3,195, and $(1,898), respectively. At December 31, 2016, the benefit obligation, fair value of plan assets, and funded status for U.S. pension plans were $4,977, $3,504, and $(1,473), respectively.
Schedule of Pension Plan Benefit Obligations	Pension Plan Benefit Obligations Pension benefits 2017 2016 The aggregate projected benefit obligation and accumulated benefit obligation for all defined benefit pension plans was as follows: Projected benefit obligation $ 7,639 $ 7,269 Accumulated benefit obligation 7,426 7,075 The aggregate projected benefit obligation and fair value of plan assets for pension plans with projected benefit obligations in excess of plan assets was as follows: Projected benefit obligation 7,061 6,699 Fair value of plan assets 4,671 4,807 The aggregate accumulated benefit obligation and fair value of plan assets for pension plans with accumulated benefit obligations in excess of plan assets was as follows: Accumulated benefit obligation 6,885 6,531 Fair value of plan assets 4,671 4,807
Components of Net Periodic Benefit Cost	Components of Net Periodic Benefit Cost Pension benefits (1) Other postretirement benefits (2) 2017 2016 2015 2017 2016 2015 Service cost $ 71 $ 61 $ 51 $ 5 $ 2 $ - Interest cost 244 138 89 38 16 4 Expected return on plan assets (398 ) (242 ) (121 ) - - - Recognized net actuarial loss 185 102 42 13 8 (3 ) Amortization of prior service cost (benefit) 9 7 6 (6 ) (5 ) (9 ) Settlements (3) 5 16 14 - - - Curtailments (4) - - 9 - - (4 ) Special termination benefits (5) 3 1 16 - - - Net periodic benefit cost (6) $ 119 $ 83 $ 106 $ 50 $ 21 $ (12 ) (1) In 2017 and 2016, net periodic benefit cost for U.S pension plans was $74 and $21, respectively. (2) In 2017 and 2016, net periodic benefit cost for other postretirement benefits reflects a reduction of $8 and $6, respectively, related to the recognition of the federal subsidy awarded under Medicare Part D. (3) In 2017 and 2016, settlements were due to workforce reductions (see Note D). In 2015, settlements were due to workforce reductions (see Note D) and the payment of lump sum benefits. (4) In 2015, curtailments were due to elimination of benefits or workforce reductions (see Note D). (5) In 2017, 2016, and 2015, special termination benefits were due to workforce reductions (see Note D). (6) Amounts attributed to joint venture partners are not included.
Schedule of Amounts Expected to be Recognized in Net Periodic Benefit Cost	Amounts Expected to be Recognized in Net Periodic Benefit Cost Pension benefits Other postretirement benefits 2018 2018 Net actuarial loss recognition $ 223 $ 15 Prior service cost (benefit) recognition 9 (6 )
Schedule of Assumed Health Care Cost Trend Rates	Assumed health care cost trend rates for U.S. other postretirement benefit plans were as follows (non-U.S. 2017 2016 2015 Health care cost trend rate assumed for next year 5.5 % 5.5 % 5.5 % Rate to which the cost trend rate gradually declines 4.5 % 4.5 % 4.5 % Year that the rate reaches the rate at which it is assumed to remain 2021 2020 2019
Schedule of One-Percentage Point Change in Assumed Rates of Health Care Cost Trend Rates	Assumed health care cost trend rates have an effect on the amounts reported for a health care plan. A one-percentage 1% increase 1% Effect on other postretirement benefit obligations $ 79 $ (70 ) Effect on total of service and interest cost components 3 (2 )
Schedule of Pension and Postretirement Plans Investment Policy and Weighted Average Asset Allocations	Alcoa Corporation’s pension plan investment policy and weighted average asset allocations at December 31, 2017 and 2016, by asset class, were as follows: Plan assets Asset class Policy range 2017 2016 Equities 20–55 % 40 % 37 % Fixed income 25–55 % 35 36 Other investments 15–35 % 25 27 Total 100 % 100 %
Schedule of Fair Value of Pension Plan Assets	The following table presents the fair value of pension plan assets classified under either the appropriate level of the fair value hierarchy or net asset value: December 31, 2017 Level 1 Level 2 Level 3 Net Asset Total Equities: Equity securities $ 906 $ - $ - $ 869 $ 1,775 Long/short equity hedge funds - - - 152 152 Private equity - - - 226 226 $ 906 $ - $ - $ 1,247 $ 2,153 Fixed income: Intermediate and long duration government/credit $ 95 $ 378 $ - $ 264 $ 737 Cash and cash equivalent funds 313 - - 742 1,055 Other - 56 - - 56 $ 408 $ 434 $ - $ 1,006 $ 1,848 Other investments: Real estate $ 241 $ - $ - $ 365 $ 606 Discretionary and systematic macro hedge funds - - - 581 581 Other - - - 132 132 $ 241 $ - $ - $ 1,078 $ 1,319 Total (1) $ 1,555 $ 434 $ - $ 3,331 $ 5,320 December 31, 2016 Level 1 Level 2 Level 3 Net Asset Total Equities: Equity securities $ 520 $ - $ - $ 772 $ 1,292 Long/short equity hedge funds - - - 449 449 Private equity - - - 246 246 $ 520 $ - $ - $ 1,467 $ 1,987 Fixed income: Intermediate and long duration government/credit $ 78 $ 283 $ - $ 167 $ 528 Cash and cash equivalent funds 897 - - 468 1,365 Other - 61 - - 61 $ 975 $ 344 $ - $ 635 $ 1,954 Other investments: Real estate $ 88 $ - $ - $ 331 $ 419 Discretionary and systematic macro hedge funds - - - 866 866 Other 71 - - 139 210 $ 159 $ - $ - $ 1,336 $ 1,495 Total (2) $ 1,654 $ 344 $ - $ 3,438 $ 5,436 (1) As of December 31, 2017, the total fair value of pension plan assets excludes a net receivable of $2, which represents securities not yet settled plus interest and dividends earned on various investments, less an amount due to Arconic pension plans from Alcoa Corporation pension plans related to the separation of certain plans between the two companies. (2) As of December 31, 2016, the total fair value of pension plan assets excludes a net payable of $15, which represents an amount due to Arconic pension plans from Alcoa Corporation pension plans related to the separation of certain plans between the two companies.
Schedule of Benefit Payments Expected to be Paid and Expected Medicare Part D Subsidy Receipts	Benefit payments expected to be paid to pension and other postretirement benefit plan participants and expected Medicare Part D subsidy receipts are as follows: Year ended December 31, Pension Gross Other Medicare Part D Net Other 2018 $ 490 $ 125 $ 10 $ 115 2019 485 125 10 115 2020 490 120 5 115 2021 490 120 5 115 2022 490 115 5 110 2023 through 2027 2,400 425 25 400 $ 4,845 $ 1,030 $ 60 $ 970
Benefit Obligation [Member]
Schedule of Total Expenses Related to All Pension and Other Postretirement Benefits	The following table summarizes the total expenses recognized by Alcoa Corporation related to all pension and other postretirement benefits: Pension benefits Other postretirement benefits Type of Plan Type of Expense 2017 2016 2015 2017 2016 2015 Cumulative Direct Plans Net periodic benefit cost $ 119 $ 83 $ 106 $ 50 $ 21 $ (12 ) Shared Plans Multiemployer contribution expense - 28 64 - 12 32 Shared Plans Cost allocation - 25 84 - 8 11 $ 119 $ 136 $ 254 $ 50 $ 41 $ 31
Schedule of Weighted Average Assumptions Used to Determine Benefit Obligations and Net Periodic Benefit Cost	Weighted average assumptions used to determine benefit obligations for pension and other postretirement benefit plans were as follows: December 31, 2017 2016 Discount rate—pension plans 3.68 % 4.12 % Discount rate—other postretirement benefit plans 3.54 3.93 Rate of compensation increase—pension plans 3.28 3.61
Net Periodic Benefit Cost [Member]
Schedule of Weighted Average Assumptions Used to Determine Benefit Obligations and Net Periodic Benefit Cost	Weighted average assumptions used to determine net periodic benefit cost for pension and other postretirement benefit plans were as follows: 2017 2016 2015 Discount rate—pension plans* 3.61 % 3.45 % 4.09 % Discount rate—other postretirement benefit plans* 3.30 2.90 4.15 Expected long-term rate of return on plan assets—pension plans 7.47 7.31 6.91 Rate of compensation increase—pension plans 3.61 3.65 3.74 * In all periods presented, the respective discount rates were used to determine net periodic benefit cost for most plans for the full annual period. However, the discount rates for a limited number of plans were updated during 2017, 2016, and 2015 to reflect the remeasurement of these plans due to settlements and/or curtailments. The updated discount rates used were not significantly different from the discount rates presented.

Derivatives and Other Financi41

Derivatives and Other Financial Instruments (Tables)	12 Months Ended
	Dec. 31, 2017
Fair Value Disclosures [Abstract]
Schedule of Quantitative Information for Level 3 Derivative Contracts	The following table presents quantitative information related to the significant unobservable inputs described above for Level 3 derivative contracts: Fair value at December 31, 2017 Unobservable input Range ($ in full amounts) Assets: Embedded aluminum derivative (D7) $ - Interrelationship of future aluminum and oil prices Aluminum: $2,258 per metric ton in January 2018 to $2,299 per metric ton in October 2018 Oil: $67 per barrel in January 2018 to $64 per barrel in October 2018 Financial contract (D11) 197 Interrelationship of forward energy price and the Consumer Price Index and price of electricity beyond forward curve Electricity: $83.69 per megawatt hour in 2018 to $53.60 per megawatt hour in 2021 Liabilities: Embedded aluminum derivative (D1) 403 Interrelationship of LME price to the amount of megawatt hours of energy needed to produce the forecasted metric tons of aluminum Aluminum: $2,258 per metric ton in 2018 to $2,651 per metric ton in 2027 Electricity: rate of 4 million megawatt hours per year Embedded aluminum derivatives (D3 through D5) 685 Price of aluminum beyond forward curve Aluminum: $2,679 per metric ton in 2028 to $2,759 per metric ton in 2029 (two contracts) and $3,055 per metric ton in 2036 (one contract) Midwest premium: $0.0950 per pound in 2018 to $0.1150 per pound in 2029 (two contracts) and 2036 (one contract) Embedded aluminum derivative (D8) 34 Interrelationship of LME price to the amount of megawatt hours of energy needed to produce the forecasted metric tons of aluminum Aluminum: $2,258 per metric ton in 2018 to $2,317 per metric ton in 2019 Midwest premium: $0.0950 per pound in 2018 to $0.1150 per pound in 2019 Electricity: rate of 2 million megawatt hours per year Embedded aluminum derivative (D2) 24 Interrelationship of LME price to overall energy price Aluminum: $2,110 per metric ton in 2018 to $2,342 per metric ton in 2019 Embedded credit derivative (D9) 27 Estimated difference in credit spread of each of Alcoa Corporation and counterparty, and negotiated multiplier 3.38% (credit spreads: Alcoa Corporation—2.48% and counterparty—1.51%; multiplier: 3.49% (1.87% 2 )
Schedule of Fair Values of Level 3 Derivative Instruments Recorded as Assets and Liabilities	The fair values of Level 3 derivative instruments recorded as assets and liabilities in the accompanying Consolidated Balance Sheet were as follows: Asset Derivatives December 31, 2017 December 31, 2016 Derivatives designated as hedging instruments: Fair value of derivative contracts—current: Embedded aluminum derivatives $ - $ 29 Financial contract 96 - Fair value of derivative contracts—noncurrent: Embedded aluminum derivatives - 468 Financial contract 101 - Total derivatives designated as hedging instruments $ 197 $ 497 Derivatives not designated as hedging instruments: Fair value of derivative contracts—current: Financial contract $ - $ 17 Total derivatives not designated as hedging instruments $ - $ 17 Total Asset Derivatives $ 197 $ 514 Liability Derivatives Derivatives designated as hedging instruments: Fair value of derivative contracts—current: Embedded aluminum derivatives $ 120 $ 17 Fair value of derivative contracts—noncurrent: Embedded aluminum derivatives 992 187 Total derivatives designated as hedging instruments $ 1,112 $ 204 Derivatives not designated as hedging instruments: Fair value of derivative contracts—current: Embedded aluminum derivative $ 28 $ 10 Embedded credit derivative 4 5 Fair value of derivative contracts—noncurrent: Embedded aluminum derivative 6 18 Embedded credit derivative 23 30 Total derivatives not designated as hedging instruments $ 61 $ 63 Total Liability Derivatives $ 1,173 $ 267
Schedule of Net Fair Values of Level 3 Derivative Instruments and Effect of Hypothetical Change (Increase or Decrease of 10%) in Market Prices or Rates	The following table shows the net fair values of the Level 3 derivative instruments at December 31, 2017 and the effect on these amounts of a hypothetical change (increase or decrease of 10%) in the market prices or rates that existed as of December 31, 2017: Fair value asset/(liability) Index change of + / -10% Embedded aluminum derivatives $ (1,146 ) $ 489 Embedded credit derivative (27 ) 3 Financial contract 197 55
Schedule of Reconciliation of Activity for Derivative Contracts	The following tables present a reconciliation of activity for Level 3 derivative contracts: Assets Liabilities 2017 Embedded aluminum derivatives Financial contracts Embedded aluminum derivatives Embedded credit derivative Opening balance—January 1, 2017 $ 497 $ 17 $ 232 $ 35 Total gains or losses (realized and unrealized) included in: Sales 3 - (110 ) - Cost of goods sold - (31 ) - (5 ) Other income, net 1 (7 ) 18 (3 ) Other comprehensive loss (499 ) 88 1,022 - Purchases, sales, issuances, and settlements* - 119 - - Transfers into and/or out of Level 3* - - - - Other (2 ) 11 (16 ) - Closing balance—December 31, 2017 $ - $ 197 $ 1,146 $ 27 Change in unrealized gains or losses included in earnings for derivative contracts held at December 31, 2017: Sales $ - $ - $ - $ - Cost of goods sold - - - - Other income, net 1 (7 ) 18 (3 ) * In January 2017, there was an issuance of a new financial contract (see D11 above). There were no purchases, sales or settlements of Level 3 derivative instruments. Additionally, there were no transfers of derivative instruments into or out of Level 3. Assets Liabilities 2016 Embedded aluminum derivatives Financial contract Embedded aluminum derivatives Embedded credit derivative Financial contract Opening balance—January 1, 2016 $ 1,135 $ 2 $ 169 $ 35 $ 4 Total gains or losses (realized and unrealized) included in: Sales (5 ) - (12 ) - - Cost of goods sold (92 ) - - (5 ) - Other income, net* (13 ) (80 ) 2 5 (2 ) Other comprehensive loss (568 ) 95 47 - (1 ) Purchases, sales, issuances, and settlements** - - 32 - - Transfers into and/or out of Level 3** - - - - - Other 40 - (6 ) - (1 ) Closing balance—December 31, 2016 $ 497 $ 17 $ 232 $ 35 $ - Change in unrealized gains or losses included in earnings for derivative contracts held at December 31, 2016: Sales $ - $ - $ - $ - $ - Cost of goods sold - - - - - Other income, net* (13 ) (80 ) 2 5 (2 ) * In August 2016, Alcoa Corporation elected to terminate the energy contract in accordance with the provisions of the agreement (see D10 above). As a result, Alcoa Corporation decreased the derivative asset and recorded a charge in Other income, net of $84, which is reflected in the table above. Additionally, Alcoa Corporation also decreased the related unrealized gain included in Accumulated other comprehensive loss and recorded a benefit in Other income, net of $84. As such, the termination of the specified term of this derivative contract described above did not have an impact on Alcoa Corporation’s earnings. ** In 2016, there was an issuance of a new embedded derivative contained in an amendment to an existing power contract (see D8 above). There were no purchases, sales or settlements of Level 3 derivative instruments. Additionally, there were no transfers of derivative instruments into or out of Level 3.
Schedule of Carrying Values and Fair Values of Other Financial Instruments	The carrying values and fair values of Alcoa Corporation’s other financial instruments were as follows: December 31, 2017 2016 Carrying Fair Carrying Fair Cash and cash equivalents $ 1,358 $ 1,358 $ 853 $ 853 Restricted cash 7 7 6 6 Long-term debt due within one year 16 16 21 21 Long-term debt, less amount due within one year 1,388 1,555 1,424 1,573

Income Taxes (Tables)

Income Taxes (Tables)	12 Months Ended
	Dec. 31, 2017
Income Tax Disclosure [Abstract]
Components of Loss from Continuing Operations Before Income Taxes	The components of income (loss) before income taxes were as follows: 2017 2016 2015 Domestic $ (712 ) $ (688 ) $ (1,053 ) Foreign 1,871 526 716 $ 1,159 $ (162 ) $ (337 )
Schedule of Provision for Income Taxes on Income from Continuing Operations	Provision for income taxes consisted of the following: 2017 2016 2015 Current: Federal* $ 3 $ 9 $ 3 Foreign 421 221 313 State and local - - - 424 230 316 Deferred: Federal* 24 - (85 ) Foreign 152 (46 ) 171 State and local - - - 176 (46 ) 86 Total $ 600 $ 184 $ 402 * Includes U.S. income taxes related to foreign income
Reconciliation of U.S. Federal Statutory Rate to Alcoa's Effective Tax Rate	A reconciliation of the U.S. federal statutory rate to Alcoa Corporation’s effective tax rate was as follows (the effective tax rate was a provision on income in 2017 and a provision on a loss in 2016 and 2015): 2017 2016 2015 U.S. federal statutory rate 35.0 % 35.0 % 35.0 % Changes in valuation allowances 25.8 (1.9 ) (62.6 ) Taxes on foreign operations—rate differential (10.8 ) 44.3 14.2 Impact of U.S. Tax Cuts and Jobs Act of 2017 1.9 - - Noncontrolling interest 1.4 (7.3 ) (8.5 ) Other taxes related to foreign operations 1.3 (19.5 ) (20.9 ) Tax holidays (1) 0.4 11.2 6.2 Losses and credits with no tax benefit (2) (0.2 ) (163.2 ) (82.0 ) Statutory tax rate and law changes 0.1 (0.6 ) (0.3 ) Nondeductible costs related to the Separation Transaction - (9.6 ) - Impact of capitalization of intercompany debt - - 3.3 Other (3.1 ) (2.0 ) (3.7 ) Effective tax rate 51.8 % (113.6 )% (119.3 )% (1) As of December 31, 2017, the income of certain operations of several of the Company’s subsidiaries in Brazil are taxed at a lower rate as a result of approved tax holidays. The difference between the respective holiday rates and the statutory rates resulted in a benefit of $20, or $0.11 per diluted share, in 2017. The majority of these tax holidays expire at the end of 2022 and one tax holiday expires at the end of 2026 (see below). In 2017, this line item also includes a charge of $26 for the remeasurement of certain deferred tax assets at the holiday rate (see below). (2) In 2016 and 2015, hypothetical net operating losses and tax credits determined on a separate return basis for which it is more likely than not that a tax benefit will not be realized. The related deferred tax asset and offsetting valuation allowance have been adjusted to Parent Company net investment and, as such, are not reflected in subsequent deferred tax and valuation allowance tables.
Schedule of Components of Net Deferred Tax Assets and Liabilities	The components of deferred tax assets and liabilities based on the underlying attribute without regard to jurisdiction were as follows: 2017 2016 December 31, Deferred tax assets Deferred tax liabilities Deferred tax assets Deferred tax liabilities Tax loss carryforwards $ 1,185 $ - $ 1,064 $ - Employee benefits 949 - 1,240 - Derivatives and hedging activities 287 70 - 124 Loss provisions 246 - 313 - Tax credit carryforwards 193 - 23 - Depreciation 141 432 187 499 Deferred income/expense 11 109 28 136 Other 100 57 233 125 3,112 668 3,088 884 Valuation allowance (1,927 ) - (1,755 ) - $ 1,185 $ 668 $ 1,333 $ 884
Schedule of Expiration Periods of Deferred Tax Assets	he following table details the expiration periods of the deferred tax assets presented above: December 31, 2017 Expires within 10 years Expires within 11-20 years No expiration* Other* Total Tax loss carryforwards $ 330 $ 250 $ 605 $ - $ 1,185 Tax credit carryforwards 193 - - - 193 Other - - 297 1,437 1,734 Valuation allowance (523 ) (152 ) (315 ) (937 ) (1,927 ) $ - $ 98 $ 587 $ 500 $ 1,185 * Deferred tax assets with no expiration may still have annual limitations on utilization. Other represents deferred tax assets whose expiration is dependent upon the reversal of the underlying temporary difference.
Composition of Net Deferred Tax Asset by Jurisdiction	The total deferred tax asset (net of valuation allowance) is supported by projections of future taxable income exclusive of reversing temporary differences and taxable temporary differences that reverse within the carryforward period. The composition of Alcoa Corporation’s net deferred tax asset by jurisdiction as of December 31, 2017 was as follows: Domestic Foreign Total Deferred tax assets $ 1,164 $ 1,948 $ 3,112 Valuation allowance (1,050 ) (877 ) (1,927 ) Deferred tax liabilities (108 ) (560 ) (668 ) $ 6 $ 511 $ 517
Schedule of Changes in Valuation Allowance	The following table details the changes in the valuation allowance: December 31, 2017 2016 2015 Balance at beginning of year $ (1,755 ) $ (712 ) $ (486 ) Establishment of new allowances (1) (94 ) - (141 ) Net change to existing allowances (2) (33 ) (1,056 ) (148 ) U.S. state tax apportionment and tax rate changes - - 30 Foreign currency translation (45 ) 13 33 Balance at end of year $ (1,927 ) $ (1,755 ) $ (712 ) (1) This line item reflects valuation allowances initially established as a result of a change in management’s judgment regarding the realizability of deferred tax assets. (2) This line item reflects movements in previously established valuation allowances, which increase or decrease as the related deferred tax assets increase or decrease. Such movements occur as a result of remeasurement due to a tax rate change and changes in the underlying attributes of the deferred tax assets, including expiration of the attribute and reversal of the temporary difference that gave rise to the deferred tax asset.
Reconciliation of Unrecognized Tax Benefits (Excluding Interest and Penalties)	A reconciliation of the beginning and ending amount of unrecognized tax benefits (excluding interest and penalties) was as follows: December 31, 2017 2016 2015 Balance at beginning of year $ 23 $ 22 $ 25 Additions for tax positions of the current year 1 3 2 Additions for tax positions of prior years - 1 1 Reductions for tax positions of prior years (5 ) (2 ) - Settlements with tax authorities (6 ) (2 ) (2 ) Expiration of the statute of limitations (3 ) - - Foreign currency translation - 1 (4 ) Balance at end of year $ 10 $ 23 $ 22

Asset Retirement Obligations (T

Asset Retirement Obligations (Tables)	12 Months Ended
	Dec. 31, 2017
Asset Retirement Obligation Disclosure [Abstract]
Schedule of Carrying Value of Recorded AROs by Major Category	The following table details the carrying value of recorded AROs by major category (of which $108 and $104 was classified as a current liability as of December 31, 2017 and 2016, respectively): December 31, 2017 2016 Mine reclamation $ 221 $ 199 Closure of bauxite residue areas 238 219 Spent pot lining disposal 125 135 Demolition* 113 121 Landfill closure 27 30 Other 1 4 $ 725 $ 708 * In 2017, 2016, and 2015, AROs were recorded as a result of management’s decision to permanently close and demolish certain structures (see Note D).
Schedule of Changes in Carrying Value of Recorded AROs	The following table details the changes in the total carrying value of recorded AROs: December 31, 2017 2016 Balance at beginning of year $ 708 $ 635 Accretion expense 17 19 Payments (69 ) (47 ) Liabilities incurred 43 117 Foreign currency translation and other 26 (16 ) Balance at end of year $ 725 $ 708

Other Financial Information (Ta

Other Financial Information (Tables)	12 Months Ended
	Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Schedule of Interest Cost Components	Interest Cost Components 2017 2016 2015 Amount charged to expense $ 104 $ 243 $ 270 Amount capitalized 17 23 30 $ 121 $ 266 $ 300
Schedule of Other (Income) Expenses, Net	Other (Income) Expenses, Net 2017 2016 2015 Equity loss $ 28 $ 70 $ 89 Foreign currency losses (gains), net 8 8 (39 ) Net gain from asset sales (116 ) (164 ) (32 ) Net loss on mark-to-market 24 9 26 Other, net (2 ) (12 ) (2 ) $ (58 ) $ (89 ) $ 42
Schedule of Other Noncurrent Assets	Other Noncurrent Assets December 31, 2017 2016 Gas supply prepayment (R) $ 510 $ 471 Value-added tax credits (1) 340 287 Prepaid gas transmission contract (H) 300 270 Goodwill (K) 154 155 Deferred mining costs, net (2) 139 127 Prepaid pension benefit (N) 72 43 Intangibles, net (K) 62 135 Other 142 180 $ 1,719 $ 1,668 (1) The Value-added tax credits relate to two of the Company’s subsidiaries in Brazil, AWAB and Alumínio. (2) As of December 31, 2016, this amount reflects an asset impairment of $72 (see Note D).
Schedule of Other Noncurrent Liabilities and Deferred Credits	Other Noncurrent Liabilities and Deferred Credits December 31, 2017 2016 Accrued compensation and retirement costs $ 127 $ 122 Deferred alumina sales revenue 68 76 Liability related to the resolution of a legal matter* - 74 Other 84 98 $ 279 $ 370 * In early 2014, ParentCo and one of Alcoa’s Corporation’s current subsidiaries, AWA, resolved violations of certain provisions of the Foreign Corrupt Practices Act of 1977 with the U.S. Department of Justice and U.S. Securities and Exchange Commission. Under the resolution, ParentCo and AWA agreed to pay a combined $384 over a four-year timeframe. Prior to the Separation Transaction, ParentCo and AWA paid $236 of the total amount. As part of the Separation and Distribution Agreement, Alcoa Corporation assumed ParentCo’s portion of the $148 remaining obligation. The $148 was paid in equal installments of $74 in January 2017 and January 2018.
Schedule of Cash Paid for Interest and Income Taxes	Cash Flow Information Cash paid for interest and income taxes was as follows: 2017 2016 2015 Interest, net of amount capitalized $ 100 $ 226 $ 270 Income taxes, net of amount refunded 363 265 265

Restructuring and Other Charg45

Restructuring and Other Charges - Schedule of Restructuring and Other Charges by Reportable Segments, Pretax (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	$ 297	$ 209	$ 309	$ 318	$ 983
Restructuring and other charges	$ 297	$ 209	309	318	983
Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			56	142	210
Restructuring and other charges			56	142	210
Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			253	176	773
Restructuring and other charges			253	176	773
Bauxite [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			2	(5)	8
Restructuring and other charges			2	(5)	8
Alumina [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			3	72	102
Restructuring and other charges			3	72	102
Aluminum Segment [Member] | Operating Segments [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			51	75	100
Restructuring and other charges			$ 51	$ 75	$ 100

Basis of Presentation - Additio

Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Millions	Dec. 31, 2017USD ($)CountryLocation$ / sharesshares	Nov. 01, 2016USD ($)$ / sharesshares	Sep. 29, 2016	Oct. 31, 2016USD ($)	Dec. 31, 2017USD ($)CountryLocation$ / shares	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)	Sep. 28, 2015
Basis Of Presentation [Line Items]
Number of countries in which entity operates | Country	10				10
Separation based on pro rata distribution percentage on common stock			80.10%
Common stock conversion ratio			0.333
Cash payment related to separation transaction		$ 1,072			$ 247	$ 1,072
Common stock distributed related to separation transaction | shares		146,159,428
Common stock par value | $ / shares	$ 0.01	$ 0.01			$ 0.01
Common stock sold | shares	36,311,767
Proceeds from sale of assets					$ 243
Costs related to separation transaction				$ 152			$ 24
Proceeds from noncontrolling shareholder					80	48	2
Sales to related parties					864	958	1,078
Outstanding receivable from related parties	$ 84				84	67
Alumina Limited [Member]
Basis Of Presentation [Line Items]
Proceeds from noncontrolling shareholder					$ 80	$ 48	2
Non-controlling interest, ownership percentage	40.00%				40.00%
Maximum [Member]
Basis Of Presentation [Line Items]
Percent of equity investments in other entity	50.00%				50.00%
Minimum [Member]
Basis Of Presentation [Line Items]
Number of operating locations | Location	40				40
Alcoa Corporation [Member]
Basis Of Presentation [Line Items]
Minority interest percentage	25.10%				25.10%			25.10%
Costs related to separation transaction				$ 68			$ 12
Ownership interest percentage	60.00%				60.00%
Parent Co [Member]
Basis Of Presentation [Line Items]
Percentage of common stock retained		19.90%	19.90%
Common stock shares retained | shares		36,311,767

Basis of Presentation - Schedul

Basis of Presentation - Schedule of Cost Allocation (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Basis Of Presentation [Line Items]
Cost of goods sold			$ 9,072	$ 7,898	$ 9,039
Selling, general administrative, and other expenses			284	359	353
Research and development expenses			32	33	69
Provision for depreciation, depletion, and amortization			750	718	780
Restructuring and other charges	$ 297	$ 209	309	318	983
Interest expense			104	243	270
Other (income) expenses, net			$ 58	89	(42)
Alcoa Corporation [Member]
Basis Of Presentation [Line Items]
Cost of goods sold				40	93
Selling, general administrative, and other expenses				150	146
Research and development expenses				2	17
Provision for depreciation, depletion, and amortization				18	22
Restructuring and other charges				1	32
Interest expense				198	245
Other (income) expenses, net				$ (7)	$ 12

Summary of Significant Accoun48

Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017USD ($)Reporting_Unit	Dec. 31, 2016USD ($)Reporting_Unit	Dec. 31, 2015USD ($)
Summary Of Significant Accounting Policies [Line Items]
Original maturity of cash equivalents	3 months
Segment Allocation, Goodwill Recognized	$ 989,000,000	$ 989,000,000
Minimum percentage of estimated fair value of reporting unit to be less than carrying amount of goodwill	50.00%
Goodwill impairment	$ 0
Maximum hedging contracts period, in years	5 years
Cost of goods sold	$ 9,072,000,000	7,898,000,000	$ 9,039,000,000
Selling, general administrative, and other expenses	284,000,000	$ 359,000,000	353,000,000
Accounting Standards Update 2017-07 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cost of goods sold	81
Selling, general administrative, and other expenses	4
Other (income) expenses, net	58
Other (income) expenses, net	27
Aluminum [Member]
Summary Of Significant Accounting Policies [Line Items]
Segment Allocation, Goodwill Recognized	$ 0
Maximum [Member] | Bauxite Mining [Member]
Summary Of Significant Accounting Policies [Line Items]
Period of mining	5 years
Minimum [Member] | Bauxite Mining [Member]
Summary Of Significant Accounting Policies [Line Items]
Period of mining	1 year
Alcoa Corporation [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units for goodwill allocation | Reporting_Unit	5	6
Cost of goods sold		$ 40,000,000	93,000,000
Selling, general administrative, and other expenses		$ 150,000,000	$ 146,000,000
Alcoa Corporation [Member] | Bauxite [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units for goodwill allocation | Reporting_Unit	1
Segment Allocation, Goodwill Recognized	$ 51,000,000
Alcoa Corporation [Member] | Alumina [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units for goodwill allocation | Reporting_Unit	1
Segment Allocation, Goodwill Recognized	$ 103,000,000
Alcoa Corporation [Member] | Aluminum [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units for goodwill allocation | Reporting_Unit	3

Summary of Significant Accoun49

Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Structures and Machinery and Equipment (Detail)	12 Months Ended
	Dec. 31, 2017
Structures [Member] | Bauxite Mining [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	35 years
Structures [Member] | Alumina Refining [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	30 years
Structures [Member] | Aluminum Smelting and Casting [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	36 years
Structures [Member] | Energy Generation [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	33 years
Structures [Member] | Aluminum Rolling [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	31 years
Machinery and Equipment [Member] | Bauxite Mining [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	17 years
Machinery and Equipment [Member] | Alumina Refining [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	28 years
Machinery and Equipment [Member] | Aluminum Smelting and Casting [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	22 years
Machinery and Equipment [Member] | Energy Generation [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	25 years
Machinery and Equipment [Member] | Aluminum Rolling [Member]
Property, Plant and Equipment [Line Items]
Weighted-average useful lives of assets, years	17 years

Summary of Significant Accoun50

Summary of Significant Accounting Policies - Weighted-Average Useful Lives of Software and Other Intangible Assets (Detail)	12 Months Ended
	Dec. 31, 2017
Software [Member] | Bauxite Mining [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	9 years
Software [Member] | Alumina Refining [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	6 years
Software [Member] | Aluminum Smelting and Casting [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	5 years
Software [Member] | Energy Generation [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	3 years
Software [Member] | Aluminum Rolling [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	4 years
Other Intangible Assets [Member] | Bauxite Mining [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	10 years
Other Intangible Assets [Member] | Alumina Refining [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	20 years
Other Intangible Assets [Member] | Aluminum Smelting and Casting [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	36 years
Other Intangible Assets [Member] | Energy Generation [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	29 years
Other Intangible Assets [Member] | Aluminum Rolling [Member]
Finite-Lived Intangible Assets [Line Items]
Weighted-average useful lives of other intangible assets	20 years

Acquisitions and Divestitures -

Acquisitions and Divestitures - Additional Information (Detail) $ in Millions	Nov. 01, 2016USD ($)	Nov. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Feb. 28, 2017USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)EmployeesHydroelectricPowerDevelopments	Dec. 31, 2014
Business Disposition [Line Items]
Cash provided at separation to Parent Company	$ 1,072				$ 247	$ 1,072
Other current liabilities					412	$ 707
Alcoa Corporation [Member]
Business Disposition [Line Items]
Percent of equity investments in other entity							60.00%
AMJ [Member]
Business Disposition [Line Items]
Percent of equity investments in other entity							55.00%
Yadkin [Member]
Business Disposition [Line Items]
Number of hydroelectric power developments | HydroelectricPowerDevelopments						4
Net cash received		$ 8	$ 8	$ 249
Cash provided at separation to Parent Company						$ 243
Sale generated						$ 29
Number of employees | Employees						30
Income (loss) from divestitures before income tax					122
Income (loss) from divestitures after income tax					$ 122
Other current liabilities						$ 243

Restructuring and Other Charg52

Restructuring and Other Charges - Schedule of Restructuring and Other Charges (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring and Related Activities [Abstract]
Early termination of a power contract			$ 244
Asset impairments			40	$ 155	$ 311
Layoff costs			23	32	199
Legal matters in Italy			(22)		201
Asset retirement obligations			10	97	76
Environmental remediation			8	26	86
Net (gain) loss on divestitures of businesses				(3)	25
Other			49	47	92
Reversals of previously recorded layoff and other costs			(43)	(36)	(7)
Restructuring and other charges	$ 297	$ 209	$ 309	$ 318	$ 983

Restructuring and Other Charg53

Restructuring and Other Charges - Schedule of Restructuring and Other Charges (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Cost and Reserve [Line Items]
Other	$ 49	$ 47	$ 92
Alcoa Corporation [Member]
Restructuring Cost and Reserve [Line Items]
Other		$ 1	$ 32

Restructuring and Other Charg54

Restructuring and Other Charges (2017 Actions) - Additional Information (Detail)	1 Months Ended		3 Months Ended								4 Months Ended	12 Months Ended
	Oct. 31, 2017USD ($)a	Jul. 31, 2017USD ($)PotlineMg	Dec. 31, 2017USD ($)Employees	Sep. 30, 2017USD ($)	Jun. 30, 2017USD ($)	Mar. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Sep. 30, 2016USD ($)	Jun. 30, 2016USD ($)	Mar. 31, 2016USD ($)	Dec. 31, 2017USD ($)Employees	Dec. 31, 2017USD ($)aEmployeeskt	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			$ 297,000,000				$ 209,000,000					$ 309,000,000	$ 318,000,000	$ 983,000,000
Cash payment												73,000,000	109,000,000	66,000,000
Sales			3,174,000,000	$ 2,964,000,000	$ 2,859,000,000	$ 2,655,000,000	2,537,000,000	$ 2,329,000,000	$ 2,323,000,000	$ 2,129,000,000		$ 11,652,000,000	9,318,000,000	11,199,000,000
Capacity closure | kt												191
Asset impairments												$ 40,000,000	155,000,000	311,000,000
Other costs												49,000,000	47,000,000	92,000,000
Asset retirement obligations												10,000,000	97,000,000	76,000,000
Environmental remediation obligations												8,000,000	26,000,000	86,000,000
Severance costs												23,000,000	32,000,000	199,000,000
Carrying value of the smelter and related assets			17,447,000,000				$ 16,741,000,000				$ 17,447,000,000	17,447,000,000	16,741,000,000
Smelting and Related Assets [Member]
Restructuring Cost and Reserve [Line Items]
Carrying value of the smelter and related assets			$ 0								0	0
Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												253,000,000	176,000,000	773,000,000
2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												$ 309,000,000
Number of employees associated with layoff costs | Employees												130
Inventory write down												$ 6,000,000
Total number of potlines | Potline		5
Total metric tons capacity of potlines | Mg		268,800
Number of potlines restarted | Potline		3
Metric tons capacity of restarted potlines | Mg		161,400
Liabilities related to original closure decision reversed												$ 33,000,000
Asset retirement obligations		$ 20,000,000
Environmental remediation obligations		4,000,000
Severance costs		$ 9,000,000
2017 Restructuring Plans Action [Member] | Smelting and Related Assets [Member]
Restructuring Cost and Reserve [Line Items]
Carrying value of the smelter and related assets										$ 0				0
2017 Restructuring Plans Action [Member] | Aluminum Segment [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employees												115
Contract Termination [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												$ 41,000,000
Cash payment	$ 238,000,000
Net asset carrying value	$ 6,000,000
Area of land transferred | a	2,200
Sales											105,000,000		141,000,000	147,000,000
Cost of goods sold											$ 148,000,000		210,000,000	201,000,000
Area of land owned | a												30,000
Capacity closure | kt												191
Asset impairments												$ 32,000,000
Cash payments made against the layoff reserves												1,000,000
Other costs												$ 16,000,000
Contract Termination [Member] | 2017 Restructuring Plans Action [Member] | Corporate [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employees												10
Asset Retirement Obligations [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												$ 10,000,000	97,000,000	76,000,000
Asset Retirement Obligations [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Other costs												8,000,000
Environmental Remediation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												8,000,000	$ 26,000,000	$ 86,000,000
Environmental Remediation [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Other costs												$ 16,000,000
Restructuring Programs Layoffs 2017 [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employees												140
Cash payment												$ 9,000,000
Approximate number of employees already laid off | Employees			90								90	90
Rarly Termination of Power Contract [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												$ 244,000,000
Exit Cost [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												49,000,000
Restructuring Programs Layoffs [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												22,000,000
Reserve Previously Established [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												22,000,000
Miscellaneous Charges [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												18,000,000
Other Adjustments [Member] | 2017 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges												$ 43,000,000

Restructuring and Other Charg55

Restructuring and Other Charges (2016 Actions) - Additional Information (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2017USD ($)kt	Dec. 31, 2016USD ($)EmployeeskT	Dec. 31, 2015USD ($)
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	$ 297	$ 209	$ 309	$ 318	$ 983
Capacity closure | kt			191
Asset impairment charges			$ 40	155	311
Severance costs			23	32	199
Other costs			49	47	92
2016 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges				$ 318
Number of employees associated with layoff costs | Employees				75
2016 Restructuring Plans Action [Member] | AofA [Member]
Restructuring Cost and Reserve [Line Items]
Percentage of ownership in gas exploration assets				43.00%
Impaired interest amount				$ 72
2016 Restructuring Plans Action [Member] | Aluminum Segment [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employees				60
2016 Restructuring Plans Action [Member] | Bauxite [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employees				15
2016 Restructuring Plans Action [Member] | Warrick Smelter, Wenatchee Smelter and Point Comfort [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges				$ 87
2016 Restructuring Plans Action [Member] | Western Australia [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges				$ 72
2016 Restructuring Plans Action [Member] | Suriname [Member]
Restructuring Cost and Reserve [Line Items]
Capacity closure | kT				2,207
2016 Restructuring Plans Action [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Other costs				$ 156
Inventory write down				5
Other related costs				4
Exit Cost [Member] | 2016 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges				131
Restructuring Programs Layoffs 2016 [Member] | 2016 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges				32
Cash payments made against the layoff reserves			2	7
Miscellaneous Charges [Member] | 2016 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges				8
Other Adjustments [Member] | 2016 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges				12
Asset Impairment and Accelerated Depreciation [Member] | 2016 Restructuring Plans Action [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges				70
Severance costs				24
Other costs				156
Shutdown and Curtailment Actions [Member] | 2016 Restructuring Plans Action [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Asset impairment charges				16
Asset Retirement Obligations [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			10	97	76
Asset Retirement Obligations [Member] | 2016 Restructuring Plans Action [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Other costs				94
Environmental Remediation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			$ 8	26	$ 86
Environmental Remediation [Member] | 2016 Restructuring Plans Action [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Other costs				26
Contract Termination [Member] | 2016 Restructuring Plans Action [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Other costs				$ 32

Restructuring and Other Charg56

Restructuring and Other Charges (2015 Actions) - Additional Information (Detail)	3 Months Ended		12 Months Ended
	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2017USD ($)kt	Dec. 31, 2016USD ($)	Dec. 31, 2015USD ($)ktEmployeeskT
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges	$ 297,000,000	$ 209,000,000	$ 309,000,000	$ 318,000,000	$ 983,000,000
Amount related to legal matters			$ (22,000,000)		201,000,000
Capacity closure | kt			191
Asset impairment charges			$ 40,000,000	155,000,000	311,000,000
2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					983,000,000
Amount related to legal matters					$ 201,000,000
Capacity of lines under review period					12 months
Capacity of lines under review | kt					2,800
Capacity of smelting lines under review | kt					500
Inventory write down					$ 90,000,000
Amount of cash payments expected to be paid beyond the end of the current annual period			4,000,000
2015 Restructuring Plans Action [Member] | Pension Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					$ 30,000,000
2015 Restructuring Plans Action [Member] | Suriname [Member]
Restructuring Cost and Reserve [Line Items]
Capacity closure | kT					1,330,000
2015 Restructuring Plans Action [Member] | Sao Luis Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Capacity closure | kT					74,000
2015 Restructuring Plans Action [Member] | TEXAS
Restructuring Cost and Reserve [Line Items]
Curtailment capacity | kT					2,010,000
Remaining curtailment capacity | kT					1,635,000
2015 Restructuring Plans Action [Member] | WASHINGTON
Restructuring Cost and Reserve [Line Items]
Curtailment capacity | kT					143,000
2015 Restructuring Plans Action [Member] | Warrick, IN Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Capacity closure | kT					269,000
2015 Restructuring Plans Action [Member] | Pocos de Caldas Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Capacity closure | kT					96
2015 Restructuring Plans Action [Member] | Point Henry Smelter [Member]
Restructuring Cost and Reserve [Line Items]
Percentage of power supply					40.00%
Exit Cost [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					$ 418,000,000
Shutdown Costs [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					238,000,000
Divested Businesses [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					24,000,000
Restructuring Programs Layoffs 2014 [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					$ 45,000,000
Number of employees associated with layoff costs | Employees					465
Asset Impairment [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					$ 33,000,000
Miscellaneous Charges [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					1,000,000
Other Adjustments [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					7,000,000
Corporate Restructuring Charges [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					32,000,000
Shutdown and Curtailment Actions [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Asset impairment charges					$ 226,000,000
Number of employees associated with layoff costs | Employees					3,100
Shutdown and Curtailment Actions [Member] | 2015 Restructuring Plans Action [Member] | Primary Metals [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employees					1,800
Shutdown and Curtailment Actions [Member] | 2015 Restructuring Plans Action [Member] | Alumina [Member]
Restructuring Cost and Reserve [Line Items]
Number of employees associated with layoff costs | Employees					1,300
Layoff Costs [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					$ 154,000,000
Asset Impairment and Accelerated Depreciation [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					85,000,000
Other Exit Costs [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					222,000,000
Asset Retirement Obligations [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			10,000,000	97,000,000	76,000,000
Asset Retirement Obligations [Member] | 2015 Restructuring Plans Action [Member] | Historical [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					72,000,000
Environmental Remediation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			8,000,000	26,000,000	86,000,000
Environmental Remediation [Member] | 2015 Restructuring Plans Action [Member] | Historical [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					85,000,000
Supplier and Customer Contract [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges					65,000,000
Restructuring Programs Layoffs 2015 [Member] | 2015 Restructuring Plans Action [Member]
Restructuring Cost and Reserve [Line Items]
Cash payments made against the layoff reserves			$ 18,000,000	$ 65,000,000	$ 26,000,000

Restructuring and Other Charg57

Restructuring and Other Charges - Activity and Reserve Balances for Restructuring Charges (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Cost and Reserve [Line Items]
Restructuring reserve beginning balance	$ 66	$ 152	$ 63
Cash payments	(73)	(109)	(66)
Restructuring charges	90	200	421
Other	(38)	(177)	(266)
Restructuring reserve ending balance	45	66	152
Layoff Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring reserve beginning balance	38	137	50
Cash payments	(30)	(74)	(65)
Restructuring charges	23	32	199
Other	(20)	(57)	(47)
Restructuring reserve ending balance	11	38	137
Other Exit Costs [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring reserve beginning balance	28	15	13
Cash payments	(43)	(35)	(1)
Restructuring charges	67	168	222
Other	(18)	(120)	(219)
Restructuring reserve ending balance	$ 34	$ 28	$ 15

Restructuring and Other Charg58

Restructuring and Other Charges - Activity and Reserve Balances for Restructuring Charges (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Restructuring Cost and Reserve [Line Items]
Pension benefits costs			$ 49	$ 47	$ 92
Restructuring and other charges	$ 297	$ 209	309	318	983
Pension and Other Postretirement Benefits Costs Charged to Restructuring [Member]
Restructuring Cost and Reserve [Line Items]
Pension benefits costs			8	16	35
Asset Retirement Obligations [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			10	97	76
Environmental Remediation [Member]
Restructuring Cost and Reserve [Line Items]
Restructuring and other charges			$ 8	$ 26	$ 86

Segment and Geographic Area I59

Segment and Geographic Area Information - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017	Sep. 28, 2015
Alcoa Corporation [Member]
Segment Reporting Information [Line Items]
Minority interest percentage	25.10%	25.10%
Alcoa Joint Venture [Member]
Segment Reporting Information [Line Items]
Ownership interest in joint venture	25.10%

Segment Information - Schedule

Segment Information - Schedule of Operating Results,Capital Expenditures and Assets of Alcoa's Reportable Segments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total sales	$ 10,788	$ 8,360	$ 10,121
Adjusted EBITDA	2,680	1,433	2,179
Depreciation, depletion, and amortization	708	677	709
Equity loss	(24)	(64)	(85)
Capital expenditures	375	394
Equity investments	1,383	1,364
Total assets	14,798	13,990
Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	14,112	11,246	13,947
Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	2,619	2,100	2,978
Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales-unrelated party	10,629	8,188	9,891
Third-party sales-related party	864	958	1,078
Bauxite [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	427	375	454
Depreciation, depletion, and amortization	82	77	93
Capital expenditures	53	29
Equity investments	191	163
Total assets	1,609	1,541
Bauxite [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	1,208	1,066	1,147
Bauxite [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	875	751	1,076
Bauxite [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales-unrelated party	333	315	71
Alumina [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	1,289	378	958
Depreciation, depletion, and amortization	207	186	192
Equity loss	(5)	(40)	(41)
Capital expenditures	144	109
Equity investments	262	342
Total assets	5,129	4,791
Alumina [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	4,856	3,607	5,068
Alumina [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	1,723	1,307	1,727
Alumina [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales-unrelated party	3,133	2,300	3,341
Aluminum [Member]
Segment Reporting Information [Line Items]
Adjusted EBITDA	964	680	767
Depreciation, depletion, and amortization	419	414	424
Equity loss	(19)	(24)	(44)
Capital expenditures	178	256
Equity investments	930	859
Total assets	8,060	7,658
Aluminum [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Total sales	8,048	6,573	7,732
Aluminum [Member] | Intersegment Eliminations [Member]
Segment Reporting Information [Line Items]
Intersegment sales	21	42	175
Aluminum [Member] | Third-Party Sales [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Third-party sales-unrelated party	7,163	5,573	6,479
Third-party sales-related party	$ 864	$ 958	$ 1,078

Segment and Geographic Area I61

Segment and Geographic Area Information - Schedule of Reconciliation of Certain Segment Information to Consolidated Totals (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting, Revenue Reconciling Item [Line Items]
Consolidated sales	$ 3,174	$ 2,964	$ 2,859	$ 2,655	$ 2,537	$ 2,329	$ 2,323	$ 2,129	$ 11,652	$ 9,318	$ 11,199
Other [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Consolidated sales									159	172	230
Operating Segments [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Consolidated sales									14,112	11,246	13,947
Intersegment Eliminations [Member]
Segment Reporting, Revenue Reconciling Item [Line Items]
Consolidated sales									$ (2,619)	$ (2,100)	$ (2,978)

Segment Information - Schedul62

Segment Information - Schedule of Segment ATOI to Combined Net Income (Loss) Attributable to Alcoa Corporation (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Segment Reporting Information [Line Items]
Total segment Adjusted EBITDA									$ 2,680	$ 1,433	$ 2,179
Metal price lag									26	9	(30)
Corporate expense									(136)	(177)	(173)
Provision for depreciation, depletion, and amortization									(750)	(718)	(780)
Interest expense									(104)	(243)	(270)
Other income (expenses), net (S)									58	89	(42)
Income (Loss) before income taxes									1,159	(162)	(337)
Provision for income taxes									(600)	(184)	(402)
Net income attributable to noncontrolling interest									(342)	(54)	(124)
Net Income (Loss) Attributable to Alcoa Corporation	$ (196)	$ 113	$ 75	$ 225	$ (125)	$ (10)	$ (55)	$ (210)	217	(400)	(863)
Other [Member]
Segment Reporting Information [Line Items]
Impact of LIFO (I)									(91)	(10)	107
Restructuring and other charges (D)									(309)	(318)	(983)
Interest expense									(104)	(243)	(270)
Other									$ (215)	$ (227)	$ (345)

Segment and Geographic Area I63

Segment and Geographic Area Information - Schedule of Segment Reporting Information to Consolidated Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule Of Assets By Segment [Line Items]
Consolidated assets	$ 17,447	$ 16,741
Cash and cash equivalents	1,358	853	$ 557	$ 266
LIFO reserve	(306)	(215)
Corporate fixed assets, net	9,138	9,325
Corporate goodwill	989	989
Deferred income taxes	517
Operating Segments [Member]
Schedule Of Assets By Segment [Line Items]
Consolidated assets	14,798	13,990
Intersegment Eliminations [Member]
Schedule Of Assets By Segment [Line Items]
Consolidated assets	(299)	(236)
Other [Member]
Schedule Of Assets By Segment [Line Items]
Cash and cash equivalents	1,358	853
LIFO reserve	(306)	(215)
Corporate fixed assets, net	520	595
Corporate goodwill	148	149
Deferred income taxes	814	741
Fair value of derivative contracts	34	497
Other	$ 380	$ 367

Segment and Geographic Informat

Segment and Geographic Information - Dchedule of Geo (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Sales Information [Line Items]
Sales	$ 3,174	$ 2,964	$ 2,859	$ 2,655	$ 2,537	$ 2,329	$ 2,323	$ 2,129	$ 11,652	$ 9,318	$ 11,199
Intersegment Eliminations [Member]
Sales Information [Line Items]
Sales									(2,619)	(2,100)	(2,978)
Primary Aluminum [Member]
Sales Information [Line Items]
Sales									6,168	5,204	6,214
Alumina [Member]
Sales Information [Line Items]
Sales									3,121	2,280	3,325
Flat-Rolled Aluminum [Member]
Sales Information [Line Items]
Sales									1,666	1,068	989
Energy [Member]
Sales Information [Line Items]
Sales									446	422	588
Bauxite [Member]
Sales Information [Line Items]
Sales									333	315	71
Other Products [Member]
Sales Information [Line Items]
Sales										$ 29	$ 12
Other Products [Member] | Intersegment Eliminations [Member]
Sales Information [Line Items]
Sales									$ (82)

Segment and Geographic Area I65

Segment and Geographic Area Information - Schedule of Geographic Information for Sales (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales	$ 3,174	$ 2,964	$ 2,859	$ 2,655	$ 2,537	$ 2,329	$ 2,323	$ 2,129	$ 11,652	$ 9,318	$ 11,199
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									5,370	4,365	5,386
Spain [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									3,303	2,663	2,852
Western Australia [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									2,266	1,644	2,147
Brazil [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									569	432	562
Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									93	141	132
Other Geographical Regions [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Sales									$ 51	$ 73	$ 120

Segment and Geographic Area I66

Segment and Geographic Area Information - Schedule of Geographic Information for Long-Lived Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 9,138	$ 9,325
Western Australia [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	2,220	2,053
Brazil [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	2,111	2,228
United States [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	1,658	1,816
Iceland [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	1,276	1,341
Canada [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	1,116	1,161
Norway [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	427	438
Spain [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	316	273
Other Geographical Regions [Member]
Revenues from External Customers and Long-Lived Assets [Line Items]
Long-lived assets	$ 14	$ 15

Earnings Per Share - Schedule o

Earnings Per Share - Schedule of Computation of Basic and Diluted EPS Attributable to Alcoa Corporation Common Shareholders (Detail) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income (loss) attributable to Alcoa Corporation	$ (196)	$ 113	$ 75	$ 225	$ (125)	$ (10)	$ (55)	$ (210)	$ 217	$ (400)	$ (863)
Average shares outstanding-basic									184,000,000	183,000,000	182,471,195
Effect of dilutive securities:
Stock options									1,000,000
Stock and performance awards									2,000,000
Average shares outstanding-diluted									187,000,000	183,000,000	182,000,000

Earnings Per Share - Additional

Earnings Per Share - Additional Information (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 01, 2016	Oct. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Common stock shares included in the calculation of EPS	184,000,000	183,000,000	182,471,195
Common stock shares issued	185,200,713	182,930,995		182,471,195	0
Common stock shares outstanding	185,200,713	182,930,995			0
Stock Awards and Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of anti-dilutive securities		1,000,000
Potential common shares that would have been included in diluted average shares outstanding		1,000,000
CL Equity Unit Purchase Agreements [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Number of anti-dilutive securities		1,000,000
Stock Options [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Weighted average exercise price of options		$ 33.05

Accumulated Other Comprehensi69

Accumulated Other Comprehensive Loss - Summary of Changes in Accumulated Other Comprehensive (Loss) Income by Component (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension and other postretirement benefits
Total Other comprehensive (loss) income	$ (447)	$ (202)	$ 80
Foreign currency translation
Other comprehensive income (loss)	284	315	(1,611)
Cash flow hedges
Total Other comprehensive (loss) income before reclassifications, net of tax	25
Total Other comprehensive (loss) income	(1,089)	(342)	826
Financial Contracts [Member]
Cash flow hedges
Total Other comprehensive (loss) income before reclassifications, net of tax		96	(2)
Alcoa Corporation [Member]
Pension and other postretirement benefits
Balance at beginning of period	(2,330)	(352)	(424)
Establishment of additional defined benefit plans		(2,704)
Separation-related adjustments		928
Unrecognized net actuarial loss and prior service cost/benefit	(671)	(307)	73
Tax benefit (expense)	25	6	(18)
Total Other comprehensive (loss) income before reclassifications, net of tax	(646)	(301)	55
Amortization of net actuarial loss and prior service cost/benefit	199	107	26
Tax benefit (expense)	(9)	(8)	(9)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	190	99	17
Total Other comprehensive (loss) income	(456)	(202)	72
Balance at end of period	(2,786)	(2,330)	(352)
Foreign currency translation
Balance at beginning of period	(1,655)	(1,851)	(668)
Separation-related adjustments		(17)
Other comprehensive income (loss)	188	213	(1,183)
Balance at end of period	(1,467)	(1,655)	(1,851)
Cash flow hedges
Balance at beginning of period	210	603	(224)
Separation-related adjustments		(47)
Net change from periodic revaluations	(1,489)	(558)	1,155
Tax benefit (expense)	251	233	(344)
Total Other comprehensive (loss) income before reclassifications, net of tax	(1,238)	(325)	811
Net amount reclassified to earnings	109	(40)	21
Tax (expense) benefit	(10)	19	(5)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	99	(21)	16
Total Other comprehensive (loss) income	(1,139)	(346)	827
Balance at end of period	(929)	210	603
Alcoa Corporation [Member] | Aluminum Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	130	7	21
Alcoa Corporation [Member] | Financial Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	(19)	(54)
Alcoa Corporation [Member] | Foreign Exchange Contract [Member]
Cash flow hedges
Net amount reclassified to earnings	(2)
Alcoa Corporation [Member] | Interest Rate Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings		7
Non-controlling Interest [Member]
Pension and other postretirement benefits
Balance at beginning of period	(56)	(56)	(64)
Unrecognized net actuarial loss and prior service cost/benefit	9	2	5
Tax benefit (expense)	(2)	(6)	(1)
Total Other comprehensive (loss) income before reclassifications, net of tax	7	(4)	4
Amortization of net actuarial loss and prior service cost/benefit	2	5	6
Tax benefit (expense)		(1)	(2)
Total amount reclassified from Accumulated other comprehensive loss, net of tax	2	4	4
Total Other comprehensive (loss) income	9		8
Balance at end of period	(47)	(56)	(56)
Foreign currency translation
Balance at beginning of period	(677)	(779)	(351)
Other comprehensive income (loss)	96	102	(428)
Balance at end of period	(581)	(677)	(779)
Cash flow hedges
Balance at beginning of period	1	(3)	(2)
Net change from periodic revaluations	83	38	(1)
Tax benefit (expense)	(25)	(12)
Total Other comprehensive (loss) income before reclassifications, net of tax	58	26	(1)
Net amount reclassified to earnings	(12)	(32)
Tax (expense) benefit	4	10
Total amount reclassified from Accumulated other comprehensive loss, net of tax	(8)	(22)
Total Other comprehensive (loss) income	50	4	(1)
Balance at end of period	51	1	$ (3)
Non-controlling Interest [Member] | Financial Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings	$ (12)	(37)
Non-controlling Interest [Member] | Interest Rate Contracts [Member]
Cash flow hedges
Net amount reclassified to earnings		$ 5

Investments - Summary of Invest

Investments - Summary of Investment (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Investments Schedule [Abstract]
Equity investments	$ 1,400	$ 1,348
Other investments	10	10
Investments	$ 1,410	$ 1,358

Investments - Schedule of Equit

Investments - Schedule of Equity Investment (Detail)	12 Months Ended
	Dec. 31, 2017
Consorcio Serra Do Facao [Member] | Brazil [Member]
Schedule of Equity Method Investments [Line Items]
Alcoa Corporation's percentage of ownership in equity investees	34.97%
Nature of equity investment	Hydroelectric generation facility
Energetica Barra Grande SA [Member] | Brazil [Member]
Schedule of Equity Method Investments [Line Items]
Alcoa Corporation's percentage of ownership in equity investees	42.18%
Nature of equity investment	Hydroelectric generation facility
Halco Mining Inc [Member] | GUINEA
Schedule of Equity Method Investments [Line Items]
Alcoa Corporation's percentage of ownership in equity investees	45.00%
Nature of equity investment	Bauxite mine
Maaden Rolling CO [Member] | SAUDI ARABIA
Schedule of Equity Method Investments [Line Items]
Alcoa Corporation's percentage of ownership in equity investees	25.10%
Nature of equity investment	Aluminum rolling mill
Maaden Aluminium Compay [Member] | SAUDI ARABIA
Schedule of Equity Method Investments [Line Items]
Alcoa Corporation's percentage of ownership in equity investees	25.10%
Nature of equity investment	Aluminum smelter
Maaden Bauxite and Alumina CO [Member] | SAUDI ARABIA
Schedule of Equity Method Investments [Line Items]
Alcoa Corporation's percentage of ownership in equity investees	25.10%
Nature of equity investment	Bauxite mine and alumina refinery
Manicouagan Power Limited Partnership [Member] | Canada [Member]
Schedule of Equity Method Investments [Line Items]
Alcoa Corporation's percentage of ownership in equity investees	40.00%
Nature of equity investment	Hydroelectric generation facility
MRN [Member] | Brazil [Member]
Schedule of Equity Method Investments [Line Items]
Alcoa Corporation's percentage of ownership in equity investees	18.20%
Nature of equity investment	Bauxite mine
Pechiney Reynolds Quebec Inc [Member] | Canada [Member]
Schedule of Equity Method Investments [Line Items]
Alcoa Corporation's percentage of ownership in equity investees	50.00%
Nature of equity investment	Aluminum smelter

Investments - Schedule of Equ72

Investments - Schedule of Equity Investment (Parenthetical) (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2014
Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest		60.00%
Halco Mining Inc [Member]
Schedule of Equity Method Investments [Line Items]
Nature of ownership	Halco Mining, Inc. owns 100% of Boké Investment Company, which owns 51% of Compagnie des Bauxites de Guinée.
Pechiney Reynolds Quebec Inc [Member]
Schedule of Equity Method Investments [Line Items]
Nature of ownership	Pechiney Reynolds Quebec, Inc. owns a 50.1% interest in the Bécancour smelter in Quebec, Canada thereby entitling Alcoa Corporation to a 25.05% interest in the smelter. Through two wholly-owned Canadian subsidiaries, Alcoa Corporation also owns 49.9% of the Bécancour smelter.
Pechiney Reynolds Quebec Inc [Member] | Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	49.90%
Boke Investment Company [Member] | Halco Mining Inc [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	100.00%
Compagnie Des Bauxites De Guinee [Member] | Halco Mining Inc [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	51.00%
Becancour Smelter [Member] | Pechiney Reynolds Quebec Inc [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	50.10%
Becancour Smelter [Member] | Pechiney Reynolds Quebec Inc [Member] | Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest	25.05%

Investments - Additional Inform

Investments - Additional Information (Detail) AUD in Millions, $ in Millions, SAR in Billions

1 Months Ended

3 Months Ended

12 Months Ended

Apr. 30, 2016USD ($)Refinery

Apr. 30, 2016AUDRefinery

Mar. 31, 2016USD ($)

Mar. 31, 2016AUD

Dec. 31, 2017USD ($)kt

Dec. 31, 2016USD ($)

Dec. 31, 2016AUD

Dec. 31, 2016SAR

Dec. 31, 2015USD ($)

Dec. 31, 2015AUD

Dec. 31, 2014USD ($)

Dec. 31, 2014AUD

Dec. 31, 2012kt

Dec. 31, 2004

Dec. 31, 2017AUD

Dec. 31, 2016AUD

Schedule of Equity Method Investments [Line Items]

Dividends from equity investments

$ 71

$ 74

$ 152

Expanded capacity amount of processed bauxite, in Kmt | kt

100

Outstanding receivable for labor and other employee-related expenses

13

11

Number of alumina refineries to be powered under supplied agreement | Refinery

3

3

Alcoa Corporation [Member]

Schedule of Equity Method Investments [Line Items]

Contribution to joint venture

$ 66

Alumina Limited [Member]

Schedule of Equity Method Investments [Line Items]

Percentage of amount required to be contributed in the event Alcoa would be required to make payments under the guarantees

40.00%

Ma'aden Joint Venture [Member]

Schedule of Equity Method Investments [Line Items]

Joint venture shareholders agreement period, years

30 years

Joint venture shareholders agreement, automatic extension additional period, years

20 years

Ownership interest in joint venture

74.90%

Contribution to joint venture

$ 199

Alcoa Joint Venture [Member]

Schedule of Equity Method Investments [Line Items]

Ownership interest in joint venture

25.10%

Maaden Alcoa Joint Venture [Member]

Schedule of Equity Method Investments [Line Items]

Ownership interest in joint venture at fair market value

14.90%

Period Bracket To Exercise Option

6 months

Period To Open Option Exercise Bracket From Commercial Production Date

5 years

Expected project investment

10,800

SAR 40.5

Capital investment

1

Equity investments

$ 887

853

Maaden Alcoa Joint Venture Mine And Refinery Company [Member] | Other Noncurrent Liabilities and Deferred Credits [Member]

Schedule of Equity Method Investments [Line Items]

Guarantee issued on behalf of smelting and rolling mill companies

3

6

Dampier to Bunbury Natural Gas Pipeline [Member] | New Equity Call Plan [Member]

Schedule of Equity Method Investments [Line Items]

Expected project investment

AUD 5

3

AUD 5

$ 16

AUD 21

$ 30

AUD 36

Capital investment

$ 20

AUD 27

Financial Guarantee [Member] | Maaden Alcoa Joint Venture Mine And Refinery Company [Member]

Schedule of Equity Method Investments [Line Items]

Debt service requirements, principal

132

Debt service requirements, interest maximum

25

Alcoa Corporation [Member] | Maaden Alcoa Joint Venture [Member]

Schedule of Equity Method Investments [Line Items]

Expected project investment

1,100

Capital investment

982

Alcoa Corporation [Member] | Maaden Alcoa Joint Venture Mine And Refinery Company [Member]

Schedule of Equity Method Investments [Line Items]

Project financing Investment

$ 3,334

Bauxite Mining [Member]

Schedule of Equity Method Investments [Line Items]

Projected capacity amount of processed bauxite, in kmt | kt

4,000

Refining Facilities [Member]

Schedule of Equity Method Investments [Line Items]

Projected capacity amount of processed bauxite, in kmt | kt

1,800

Primary Aluminum Smelter [Member]

Schedule of Equity Method Investments [Line Items]

Projected capacity amount of processed bauxite, in kmt | kt

740

Rolling Mill [Member]

Schedule of Equity Method Investments [Line Items]

Projected capacity amount of processed bauxite, in kmt | kt

380

AWAC [Member] | Maaden Alcoa Joint Venture Mine And Refinery Company [Member]

Schedule of Equity Method Investments [Line Items]

Project financing Investment

$ 837

AofA [Member]

Schedule of Equity Method Investments [Line Items]

Investment percentage in DBP to be sold

20.00%

20.00%

Amount to be received by sale of investment percentage in DBP

$ 145

AUD 192

Gain after tax and noncontrolling interest

11

15

Pretax gain on sale of investments

27

AUD 35

AofA [Member] | Dampier to Bunbury Natural Gas Pipeline [Member]

Schedule of Equity Method Investments [Line Items]

Investment percentage

30.00%

30.00%

20.00%

Prepayments made under the agreement for future gas transmission services

$ 300

$ 270

AUD 385

AUD 375

Investments - Summary of Financ

Investments - Summary of Financial Information for Alcoa Corporation's Equity Investments (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Schedule of Equity Method Investments [Line Items]
Sales	$ 4,469	$ 3,507	$ 4,119
Cost of goods sold	3,781	2,844	3,209
(Loss) Income before income taxes	132	85	125
Net (loss) income	82	21	43
Equity in net (loss) income of affiliated companies, before reconciling adjustments	52	25	60
Other	10	3	(7)
Alcoa Corporation's equity in net (loss) income of affiliated companies	62	28	53
Current assets	1,862	1,396
Noncurrent assets	11,015	11,454
Current liabilities	1,699	1,767
Noncurrent liabilities	6,751	6,777
Ma'aden Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Sales	3,032	1,970	2,025
Cost of goods sold	2,776	1,905	2,070
(Loss) Income before income taxes	(142)	(295)	(362)
Net (loss) income	(157)	(295)	(366)
Equity in net (loss) income of affiliated companies, before reconciling adjustments	(39)	(75)	(91)
Other	9	7	(2)
Alcoa Corporation's equity in net (loss) income of affiliated companies	(30)	(68)	(93)
Current assets	1,415	1,141
Noncurrent assets	9,373	9,653
Current liabilities	1,331	1,452
Noncurrent liabilities	6,191	6,204
Halco Mining [Member]
Schedule of Equity Method Investments [Line Items]
Sales	416	437	487
Cost of goods sold	266	242	236
(Loss) Income before income taxes	50	53	86
Net (loss) income	47	50	80
Equity in net (loss) income of affiliated companies, before reconciling adjustments	21	23	36
Other	(1)	2	(2)
Alcoa Corporation's equity in net (loss) income of affiliated companies	20	25	34
Current assets	40	12
Noncurrent assets	174	189
Current liabilities	1	4
Noncurrent liabilities	12	14
Energetica Barra Grande SA [Member]
Schedule of Equity Method Investments [Line Items]
Sales	131	60	130
Cost of goods sold	117	35	98
(Loss) Income before income taxes	13	16	27
Net (loss) income	5	15	7
Equity in net (loss) income of affiliated companies, before reconciling adjustments	2	6	3
Other		(1)	(1)
Alcoa Corporation's equity in net (loss) income of affiliated companies	2	5	2
Current assets	44	17
Noncurrent assets	285	302
Current liabilities	48	31
Noncurrent liabilities	6	23
Pechiney Reynolds Quebec Inc [Member]
Schedule of Equity Method Investments [Line Items]
Sales	332	309	486
Cost of goods sold	292	272	288
(Loss) Income before income taxes	35	36	113
Net (loss) income	23	16	104
Equity in net (loss) income of affiliated companies, before reconciling adjustments	11	8	52
Other		(4)	4
Alcoa Corporation's equity in net (loss) income of affiliated companies	11	4	56
Current assets	140	91
Noncurrent assets	106	147
Current liabilities	65	62
Noncurrent liabilities	12
Consorcio Serra Do Facao [Member]
Schedule of Equity Method Investments [Line Items]
Sales	103	90	84
Cost of goods sold	48	65	76
(Loss) Income before income taxes	45	8	6
Net (loss) income	46	5	(1)
Equity in net (loss) income of affiliated companies, before reconciling adjustments	16	2
Other			(2)
Alcoa Corporation's equity in net (loss) income of affiliated companies	16	2	(2)
Current assets	79	19
Noncurrent assets	261	451
Current liabilities	25	37
Noncurrent liabilities	117	283
MRN [Member]
Schedule of Equity Method Investments [Line Items]
Sales	350	451	498
Cost of goods sold	273	297	308
(Loss) Income before income taxes	35	152	118
Net (loss) income	30	129	98
Equity in net (loss) income of affiliated companies, before reconciling adjustments	6	23	18
Other		(1)	(3)
Alcoa Corporation's equity in net (loss) income of affiliated companies	6	22	15
Current assets	121	92
Noncurrent assets	744	644
Current liabilities	220	174
Noncurrent liabilities	413	253
Manicouagan Power Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Sales	105	104	106
Cost of goods sold	9	10	16
(Loss) Income before income taxes	96	94	91
Net (loss) income	88	87	90
Equity in net (loss) income of affiliated companies, before reconciling adjustments	35	35	36
Other	2
Alcoa Corporation's equity in net (loss) income of affiliated companies	37	35	36
Current assets	23	24
Noncurrent assets	72	68
Current liabilities	$ 9	7
Dampier to Bunbury Natural Gas Pipeline [Member]
Schedule of Equity Method Investments [Line Items]
Sales		86	303
Cost of goods sold		18	117
(Loss) Income before income taxes		21	46
Net (loss) income		14	31
Equity in net (loss) income of affiliated companies, before reconciling adjustments		3	6
Other			(1)
Alcoa Corporation's equity in net (loss) income of affiliated companies		$ 3	$ 5

Inventories - Schedule of Inven

Inventories - Schedule of Inventory Components (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Inventory Disclosure [Abstract]
Finished goods	$ 296	$ 226
Work-in-process	258	220
Bauxite and alumina	585	429
Purchased raw materials	473	363
Operating supplies	147	137
LIFO reserve	(306)	(215)
Inventories, total	$ 1,453	$ 1,160

Inventories - Additional Inform

Inventories - Additional Information (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Inventory Disclosure [Abstract]
Inventories valued on a LIFO basis	$ 516	$ 393
Inventories valued on a LIFO basis percentage	29.00%	29.00%

Properties, Plants, and Equip77

Properties, Plants, and Equipment, Net - Schedule of Properties, Plants, and Equipment, Net (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	$ 9,138	$ 9,325
Less: accumulated depreciation, depletion, and amortization	13,908	13,225
Properties, plants, and equipment excluding construction work-in-progress	8,751	8,827
Construction work-in-progress	387	498
Properties, plants, and equipment, net	9,138	9,325
Land [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	346	346
Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	8,756	8,540
Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	13,557	13,166
Property Plant And Equipment Other Than Construction In Progress [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	22,659	22,052
Operating Segments [Member] | Bauxite Mining [Member] | Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	1,250	1,194
Operating Segments [Member] | Bauxite Mining [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	508	465
Operating Segments [Member] | Alumina Refining [Member] | Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	2,664	2,500
Operating Segments [Member] | Alumina Refining [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	4,009	3,773
Operating Segments [Member] | Aluminum Smelting and Casting [Member] | Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	3,575	3,544
Operating Segments [Member] | Aluminum Smelting and Casting [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	6,827	6,655
Operating Segments [Member] | Energy [Member] | Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	552	610
Operating Segments [Member] | Energy [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	905	1,080
Operating Segments [Member] | Aluminum Rolling [Member] | Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	298	287
Operating Segments [Member] | Aluminum Rolling [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	1,020	884
Other [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, net	520	595
Other [Member] | Structures [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	417	405
Other [Member] | Machinery and Equipment [Member]
Property, Plant and Equipment [Line Items]
Properties, plants, and equipment, gross	$ 288	$ 309

Properties, Plants, and Equip78

Properties, Plants, and Equipment, Net - Additional Information (Detail)	1 Months Ended	12 Months Ended
	Jul. 31, 2017kt	Dec. 31, 2017USD ($)kt	Dec. 31, 2016USD ($)kt
Property, Plant and Equipment [Line Items]
Refining capacity closed		191
Refining ideal capacity closed		778
Partially started smelter capacity	161
Carrying value of the smelter and related assets | $		$ 17,447,000,000	$ 16,741,000,000
Smelting Assets [Member]
Property, Plant and Equipment [Line Items]
Net carrying value of assets | $		$ 248,000,000	$ 314,000,000
Idle capacity of assets, units	269	856	778
Idled Refining Assets [Member]
Property, Plant and Equipment [Line Items]
Net carrying value of assets | $		$ 141,000,000	$ 158,000,000
Idle capacity of assets, units		2,305	2,305
Smelting and Related Assets [Member]
Property, Plant and Equipment [Line Items]
Carrying value of the smelter and related assets | $		$ 0

Goodwill and Other Intangible79

Goodwill and Other Intangible Assets - Summary of Goodwill which is Included in Other Noncurret Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill	$ 989	$ 989
Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	154	155
Bauxite [Member] | Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	2	2
Alumina [Member] | Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	4	4
Corporate Segment [Member] | Other Noncurrent Assets [Member]
Goodwill [Line Items]
Goodwill	$ 148	$ 149

Goodwill and Other Intangible80

Goodwill and Other Intangible Assets - Summary of Goodwill which is Included in Other Noncurret Assets (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Goodwill [Line Items]
Goodwill	$ 989	$ 989
Bauxite [Member]
Goodwill [Line Items]
Segment reporting, goodwill	49
Alumina [Member]
Goodwill [Line Items]
Segment reporting, goodwill	99
Corporate Segment [Member]
Goodwill [Line Items]
Accumulated impairment losses	742	742
Aluminum [Member]
Goodwill [Line Items]
Goodwill	0
Accumulated impairment losses	$ 989	$ 989

Goodwill and Other Intangible81

Goodwill and Other Intangible Assets - Other Intangible Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Finite-Lived Intangible Assets [Line Items]
Total amortizable intangible assets, Gross carrying amount	$ 287	$ 343
Total amortizable intangible assets, Accumulated amortization	(225)	(208)
Software [Member]
Finite-Lived Intangible Assets [Line Items]
Total amortizable intangible assets, Gross carrying amount	241	252
Total amortizable intangible assets, Accumulated amortization	(212)	(196)
Patent and Licenses [Member]
Finite-Lived Intangible Assets [Line Items]
Total amortizable intangible assets, Gross carrying amount	25	70
Total amortizable intangible assets, Accumulated amortization	(6)	(6)
Other Intangible Assets [Member]
Finite-Lived Intangible Assets [Line Items]
Total amortizable intangible assets, Gross carrying amount	21	21
Total amortizable intangible assets, Accumulated amortization	$ (7)	$ (6)

Goodwill and Other Intangible82

Goodwill and Other Intangible Assets - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Intangible Liability Disclosure [Abstract]
Amortization expense related to the intangible assets	$ 12,000,000	$ 7,000,000	$ 10,000,000
Expected amortization for the year 2018	10,000,000
Expected amortization for the year 2019	10,000,000
Expected amortization for the year 2020	10,000,000
Expected amortization for the year 2021	10,000,000
Expected amortization for the year 2022	$ 10,000,000

Debt - Schedule of Long-Term De

Debt - Schedule of Long-Term Debt (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Unamortized discounts and deferred financing costs	$ (33)	$ (35)
Long-term debt	1,404	1,445
Long-term debt	1,404	1,445
Less: amount due within one year	16	21
Long-term debt non current	1,388	1,424
6.75% Notes, due 2024 [Member]
Debt Instrument [Line Items]
Long-term debt	750	750
7.00% Notes, due 2026 [Member]
Debt Instrument [Line Items]
Long-term debt	500	500
BNDES Loans Due Two 2018-2029 [Member]
Debt Instrument [Line Items]
Long-term debt	137	192
Other [Member]
Debt Instrument [Line Items]
Long-term debt	$ 50	$ 38

Debt - Schedule of Long-Term 84

Debt - Schedule of Long-Term Debt (Parenthetical) (Detail) - BNDES Loans, due 2017-2029 [Member]	12 Months Ended
	Dec. 31, 2017
Debt Instrument [Line Items]
Debt instrument, maturity date start	2,018
Debt instrument, maturity date end	2,029

Debt - Principal maturities of

Debt - Principal maturities of long-term debt - Additional Information (Detail) $ in Millions	Dec. 31, 2017USD ($)
Debt Disclosure [Abstract]
Principal amount of long-term debt maturing in year 2018	$ 16
Principal amount of long-term debt maturing in year 2019	30
Principal amount of long-term debt maturing in year 2020	15
Principal amount of long-term debt maturing in year 2021	15
Principal amount of long-term debt maturing in year 2022	$ 15

Debt - Additional Information (

Debt - Additional Information (Detail) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Principal amount of debt		$ 1,404,000,000	$ 1,445,000,000
Proceeds from issuance of public debt offering		21,000,000
Alcoa Nederland Holding BV [Member]
Debt Instrument [Line Items]
Proceeds from issuance of public debt offering	$ 1,228,000,000
Cash on hand to fund redemption of senior notes		$ 81,000,000
6.75% Senior Notes Due 2024 [Member] | Alcoa Nederland Holding BV [Member]
Debt Instrument [Line Items]
Principal amount of debt	$ 750,000,000
Senior notes, interest percentage	6.75%
6.75% Senior Notes Due 2024 [Member] | Alcoa Nederland Holding BV [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument redemption price percentage		105.063%
7% Senior Notes Due 2026 [Member]
Debt Instrument [Line Items]
Debt convenant percentage of assets		1.50%
7% Senior Notes Due 2026 [Member] | Debt Covenant Terms November 1, 2016 through December 31, 2017 [Member]
Debt Instrument [Line Items]
Amount of dividend restriction		$ 38,000,000
7% Senior Notes Due 2026 [Member] | Debt Covenant Terms 2018 [Member]
Debt Instrument [Line Items]
Amount of dividend restriction		38,000,000
7% Senior Notes Due 2026 [Member] | Debt Covenant Terms 2019 [Member]
Debt Instrument [Line Items]
Amount of dividend restriction		0.50
7% Senior Notes Due 2026 [Member] | Debt Covenant Terms 2020 [Member]
Debt Instrument [Line Items]
Amount of dividend restriction		0.50
7% Senior Notes Due 2026 [Member] | Debt Covenant Terms January One Two Thousand And Twenty One Through September Thirty Two Thousand And Twenty Six [Member]
Debt Instrument [Line Items]
Amount of dividend restriction		75,000,000
7% Senior Notes Due 2026 [Member] | Minimum [Member]
Debt Instrument [Line Items]
Amount of dividend restriction		$ 250,000,000
7% Senior Notes Due 2026 [Member] | Alcoa Nederland Holding BV [Member]
Debt Instrument [Line Items]
Principal amount of debt	$ 500,000,000
Senior notes, interest percentage	7.00%
Debt instrument maturity date	2,026
7% Senior Notes Due 2026 [Member] | Alcoa Nederland Holding BV [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument redemption price percentage		103.50%
Notes 2024 and 2026 [Member] | Alcoa Nederland Holding BV [Member]
Debt Instrument [Line Items]
Debt instrument maturity date	2,024
Debt redemption description		ANBHV has the option to redeem the Notes on at least 30 days, but not more than 60 days, prior notice to the holders of the Notes under multiple scenarios, including, in whole or in part, at any time or from time to time after September 2019, in the case of the 2024 Notes, or after September 2021, in the case of the 2026 Notes, at a redemption price specified in the indenture (up to 105.063% of the principal amount for the 2024 Notes and up to 103.500% of the principal amount of the 2026 Notes, plus any accrued and unpaid interest in each case).
Debt instrument redemption price percentage		101.00%
Cash held in escrow		$ 1,228,000,000
Notes 2024 and 2026 [Member] | Alcoa Nederland Holding BV [Member] | Minimum [Member]
Debt Instrument [Line Items]
Debt instrument redemption period		30 days
Notes 2024 and 2026 [Member] | Alcoa Nederland Holding BV [Member] | Maximum [Member]
Debt Instrument [Line Items]
Debt instrument redemption period		60 days

Debt (BNDES) - Additional Infor

Debt (BNDES) - Additional Information (Detail) - Alcoa Aluminio [Member]	1 Months Ended		12 Months Ended
	Aug. 31, 2017USD ($)	Aug. 31, 2017BRL	Dec. 31, 2017USD ($)Agreement	Dec. 31, 2017BRLAgreement	Dec. 31, 2016USD ($)	Dec. 31, 2016BRL	Dec. 31, 2017BRL	Dec. 31, 2016BRL
Debt Instrument [Line Items]
Commitment on loan agreement			$ 85,000,000				BRL 177,000,000
Weighted-average interest rate			974.00%	974.00%	1022.00%	1022.00%
Early repayment of outstanding borrowings			$ 44,000,000	BRL 138,000,000	$ 3,000,000	BRL 11,000,000
Interest rate margin			1.55%	1.55%
Outstanding borrowings					$ 42,000,000			BRL 137,000,000
Interest rate of outstanding borrowings					9.05%	9.05%
Early payment	$ 41,000,000	BRL 131,000,000
Loan Agreement With Bndes [Member]
Debt Instrument [Line Items]
Long-term interest rate			7.00%		7.50%		7.00%	7.50%
Weighted-average margin on long term debt			2.74%	2.74%	2.74%	2.74%
Number of Subloan agreement | Agreement			3	3
Three Of Subloans [Member] | Loan Agreement With Bndes [Member]
Debt Instrument [Line Items]
Commitment on loan agreement			$ 397,000,000				BRL 687,000,000
Two Of Subloans [Member] | Loan Agreement With Bndes [Member]
Debt Instrument [Line Items]
Principal and interest							667,000,000
Third Subloan [Member] | Loan Agreement With Bndes [Member]
Debt Instrument [Line Items]
Principal and interest							20,000,000
BNDES Loans, Due 2014-2029 [Member]
Debt Instrument [Line Items]
Line of credit facility, outstanding borrowings			137		$ 150		BRL 454	BRL 490
Early repayment of outstanding borrowings			$ 15	BRL 49	$ 14	BRL 48

Debt (Credit Facility) - Additi

Debt (Credit Facility) - Additional Information (Detail) - USD ($)	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Sep. 16, 2016
Debt Instrument [Line Items]
Principal amount of debt	$ 1,445,000,000	$ 1,404,000,000	$ 1,445,000,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Line of credit facility, outstanding borrowings	0	$ 0	$ 0	$ 1,500,000,000
Line of credit facility, maturity date		Nov. 14, 2022
Percentage of equity interest in foreign subsidiaries pledged as security to secured debt		65.00%
Revolving credit facility, covenant description		The Amended Revolving Credit Agreement also includes financial covenants requiring the maintenance of a specified interest expense coverage ratio of not less than 5.00 to 1.00, and a leverage ratio for any period of four consecutive fiscal quarters that is not greater than 2.25 to 1.00 (may be increased to a level not higher than 2.50 to 1.00). As of December 31, 2017 and 2016, Alcoa Corporation was in compliance with all such covenants.
Leverage ratio, covenants requirements		1.20%	1.30%
Incremental amount of convenant, maximum	500,000,000	$ 1,000,000,000	$ 500,000,000
Debt convenant percentage of assets		6.00%	3.00%
Restricted payments negative covenant, maximum	0	$ 100,000,000	$ 0
Investment negative covenant, maximum	250,000,000	400,000,000	250,000,000
Thresholds for restricted payments negative covenants	0	250,000,000	0
Thresholds for investments negative covenants	0	$ 200,000,000	$ 0
Percentage of unused base amount used in the succeeding fiscal year		50.00%
Percentage of unused portion of credit facility		50.00%
Annual share repurchase limit		$ 100,000,000
Amounts borrowed under the credit facility	$ 0	$ 0
Revolving Credit Facility [Member] | Maximum [Member]
Debt Instrument [Line Items]
Leverage ratio, covenants requirements		225.00%
Leverage ratio, maximum possible increase		250.00%
Annual share repurchase limit		$ 25,000,000
Amount of dividend restriction		$ 100,000,000
Revolving Credit Facility [Member] | Minimum [Member]
Debt Instrument [Line Items]
Interest expense coverage ratio required to be maintained		5
Revolving Credit Facility [Member] | Debt Covenant Terms November 1, 2016 through December 31, 2017 [Member]
Debt Instrument [Line Items]
Amount of dividend restriction		$ 38,000,000
Revolving Credit Facility [Member] | Debt Covenant Terms 2018 [Member]
Debt Instrument [Line Items]
Amount of dividend restriction		38,000,000
Revolving Credit Facility [Member] | Debt Covenant Terms 2019 [Member]
Debt Instrument [Line Items]
Amount of dividend restriction		50,000,000
Revolving Credit Facility [Member] | Debt Covenant Terms 2020 [Member]
Debt Instrument [Line Items]
Amount of dividend restriction		50,000,000
Revolving Credit Facility [Member] | Debt Covenant Terms January 01, 2021 through November 01, 2021 [Member]
Debt Instrument [Line Items]
Amount of dividend restriction		$ 75,000,000
Revolving Credit Facility [Member] | Alcoa Nederland Holding BV [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum outstanding borrowings				750,000,000
Letters of credit sublimit under credit facility				400,000,000
Credit facility, interest rate description		Loans will bear interest at a rate per annum equal to an applicable margin plus, at ANHBV’s option, either (a) an adjusted LIBOR rate or (b) a base rate determined by reference to the highest of (1) the prime rate of JPMorgan Chase Bank, N.A., (2) the greater of the federal funds effective rate and the overnight bank funding rate, plus 0.5%, and (3) the one month adjusted LIBOR rate plus 1% per annum.
Revolving Credit Facility [Member] | Alcoa Nederland Holding BV [Member] | Federal Funds Effective Rate [Member]
Debt Instrument [Line Items]
Debt instrument, margin over variable rate		0.50%
Revolving Credit Facility [Member] | Alcoa Nederland Holding BV [Member] | LIBOR [Member]
Debt Instrument [Line Items]
Debt instrument, margin over variable rate		1.00%
Revolving Credit Facility [Member] | Alcoa Nederland Holding BV [Member] | Maximum [Member]
Debt Instrument [Line Items]
Principal amount of debt		$ 500,000,000		0
Commitment fee paid to maintain credit facility		0.45%
Applicable margin on LIBOR loans		2.50%
Applicable margin on base rate loans		1.50%
Revolving Credit Facility [Member] | Alcoa Nederland Holding BV [Member] | Minimum [Member]
Debt Instrument [Line Items]
Commitment fee paid to maintain credit facility		0.225%
Applicable margin on LIBOR loans		1.75%
Applicable margin on base rate loans		0.75%
Revolving Credit Facility [Member] | Alcoa Nederland Holding BV [Member] | Letter of Credit [Member]
Debt Instrument [Line Items]
Line of credit facility, maximum outstanding borrowings				$ 750,000,000

Preferred and Common Stock - Ad

Preferred and Common Stock - Additional Information (Detail) - USD ($)	Nov. 01, 2016	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Oct. 31, 2016
Class of Stock [Line Items]
Preferred stock, authorized (in shares)			100,000,000
Preferred stock, par value (in dollars per share)			$ 0.01
Preferred stock, issued (in shares)		0	0	0
Common stock shares authorized			750,000,000
Common stock par value	$ 0.01		$ 0.01
Common stock shares issued	182,471,195	182,930,995	185,200,713	182,930,995		0
Common stock distributed related to separation transaction	146,159,428
Common stock shares retained by ParentCo	36,311,767
Common stock share outstanding		182,930,995	185,200,713	182,930,995		0
Common stock dividends declared during period		$ 0
Common stock were reserved for future issuance			27,270,482
Increase in exercise price	$ 1.34
Adjusted exercise option price	$ 28.72		$ 21.44
Stock based compensation expense			$ 24,000,000	$ 28,000,000	$ 35,000,000
Stock based compensation expense, stock awards percentage			90.00%	90.00%	90.00%
Expense related to the acceleration of expense related to retirement-eligible employees			$ 4,000,000	$ 7,000,000	$ 6,000,000
Volatility			40.68%
Average risk-free interest rate			2.48%
Fair value of stock options granted			$ 12.45	$ 2.12	$ 4.47
Dividend yield			0.00%
Annual pre- and post-vesting forfeitures			6.00%
Exercise behavior			59.00%
Life (years)			5 years 8 months 12 days
Description of reverse stock split			ParentCo's stock-based compensation plan to Alcoa Corporation employees, the previously-mentioned fair values were adjusted to reflect both the impact of ParentCo's 1-for-3 reverse stock split that occurred on October 5, 2016 and to maintain the intrinsic value of the stock options as a result of the Separation Transaction.
Number of options, Outstanding weighted average remaining contractual life			5 years 9 months 25 days
Total intrinsic value of options outstanding			$ 76,000,000
Stock options vested			1,704,299
Stock option exercisable			1,704,299
Weighted average exercise price, vested and exercisable			$ 26.19
Total intrinsic value of options vested and exercisable		$ 47,000,000		$ 47,000,000
Cash received from option exercises			$ 43,000,000	10,000,000
Total intrinsic value of options exercised			24,000,000	$ 4,000,000
Unrecognized compensation costs on non-vested stock option grants (pretax)			$ 24,000,000
Unrecognized compensation costs on non-vested awards, weighted average period of recognition in years			1 year 9 months
Minimum [Member]
Class of Stock [Line Items]
Stock based compensation expense, stock awards percentage			80.00%	80.00%	80.00%
Common Stock [Member]
Class of Stock [Line Items]
Shares issued for employee stock-based compensation plans		459,800	2,269,718
Alcoa Corporation [Member]
Class of Stock [Line Items]
Fair value of stock options granted				$ 4.75	$ 10.01
Employees [Member]
Class of Stock [Line Items]
Stock based compensation expense				$ 16,000,000	$ 21,000,000
Stock Awards [Member]
Class of Stock [Line Items]
Number of awards, Outstanding	2,605,423	2,557,842	2,301,332	2,557,842
Stock granted, Fair value			$ 37.39
Restricted Stock [Member]
Class of Stock [Line Items]
Stock granted, Fair value			$ 52.01
Volatility			36.98%
Average risk-free interest rate			1.44%
Restricted Stock [Member] | Standard & Poor's 500 [Member]
Class of Stock [Line Items]
Volatility			11.44%
Stock Options [Member]
Class of Stock [Line Items]
Adjusted exercise option price		$ 24.69	$ 25.48	$ 24.69
Number of options, Outstanding	4,673,829	4,108,535	2,687,107	4,108,535
Stock option exercisable			1,679,148

Preferred and Common Stock - Sc

Preferred and Common Stock - Schedule of Activity for Stock Options and Stock Awards (Detail)	12 Months Ended
	Dec. 31, 2017$ / sharesshares
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Number of options, Exercised | shares	(1,704,299)
Weighted average exercise price Outstanding, end of year | $ / shares	$ 21.44
Stock Options [Member]
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Number of options, Outstanding beginning of year | shares	4,108,535
Number of options, Granted | shares	331,681
Number of options, Exercised | shares	(1,679,148)
Converted | shares	0
Number of options, Expired or forfeited | shares	(73,961)
Number of options, Performance share adjustment | shares	0
Number of options, Outstanding end of year | shares	2,687,107
Weighted average exercise price, Outstanding beginning of year | $ / shares	$ 24.69
Weighted average exercise price, granted | $ / shares	37.68
Weighted average exercise price, exercised | $ / shares	25.80
Weighted average exercise price, Converted | $ / shares	0
Weighted average exercise price, Expired or forfeited | $ / shares	28.99
Weighted average exercise price, Performance share adjustment | $ / shares	0
Weighted average exercise price Outstanding, end of year | $ / shares	$ 25.48
Stock Awards [Member]
Compensation Related Costs Share Based Payments Disclosure [Line Items]
Number of awards, Outstanding beginning of year | shares	2,557,842
Number of awards, Granted | shares	796,330
Number of awards, Exercised | shares	0
Number of awards, Converted | shares	(764,064)
Number of awards, Expired or forfeited | shares	(145,756)
Number of awards, Performance share adjustment | shares	(143,020)
Number of awards, Outstanding end of year | shares	2,301,332
Weighted average FMV per award, Outstanding beginning of year | $ / shares	$ 22.65
Weighted average FMV per award, Granted | $ / shares	37.39
Weighted average FMV per award, Exercised | $ / shares	0
Weighted average FMV per award, Converted | $ / shares	24.98
Weighted average FMV per award, Expired or forfeited | $ / shares	24.90
Weighted average FMV per award, Performance share adjustment | $ / shares	21.03
Weighted average FMV per award, Outstanding, end of year | $ / shares	$ 26.93

Pension and Other Postretirem91

Pension and Other Postretirement Benefits - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 01, 2016
Defined Benefit Plan Disclosure [Line Items]
Percentage increase in rate of compensation	3.28%	3.61%
Defined benefit plan, description of basis used to determine overall expected long-term rate-of-return on assets assumption	The expected long-term rate of return on plan assets is generally applied to a five-year market-related value of plan assets (a four-year average or the fair value at the plan measurement date is used for certain non-U.S.plans)
Expected long term rate of return on plan assets	6.89%
Health care cost trend rate assumed for next year	5.50%	5.50%	5.50%
Health care cost trend rate	4.50%	4.50%	4.50%
Number of years over actual annual health care cost trend experience	3 years
Cash contribution to pension plans	$ 106,000,000	$ 66,000,000	$ 69,000,000
Expected minimum required cash contribution to pension plans, next year	290,000,000
Expenses related to saving and investment plans	$ 65,000,000	$ 57,000,000	$ 59,000,000
Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected asset class mix percentage	20.00%
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected asset class mix percentage	30.00%
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, expected asset class mix percentage	50.00%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate used to determine pension benefit obligation	3.68%	4.12%
Pension and other postretirement benefit plan current liability	$ 11,000,000	$ 10,000,000
Pension and other postretirement benefit plan noncurrent liability	$ 2,341,000,000	$ 1,851,000,000
Percentage increase in rate of compensation	3.28%
Expected long term rate of return on plan assets	7.47%	7.31%	6.91%
Expenses related to saving and investment plans		$ 25,000,000	$ 84,000,000
Domestic Plan [Member] | United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Expected minimum required cash contribution to pension plans, next year	$ 250,000,000
Domestic Plan [Member] | US and Canadian [Member]
Defined Benefit Plan Disclosure [Line Items]
Discretionary contributions	$ 300,000,000
Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Health care cost trend rate	9.00%
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Average duration refined yield curve model parallels the plans, years		12 years
Health care cost trend rate	(0.80%)
New Direct Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate used to determine pension benefit obligation				3.48%
Pension and other postretirement benefit plan unfunded status				$ 2,348,000,000
Pension and other postretirement benefit plan current liability				136,000,000
Pension and other postretirement benefit plan noncurrent liability				2,392,000,000
Pension and other postretirement benefit plan accumulated other comprehensive loss				$ 2,704,000,000

Pension and Other Postretirem92

Pension and Other Postretirement Benefits - Total Expenses Recognized to All Pension and Other Postretirement Benefits (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Contribution Plan Disclosure [Line Items]
Cumulative Direct Plans - Net periodic benefit cost	$ 74	$ 21
Shared Plans - Cost allocation	65	57	$ 59
Pension Benefits [Member]
Defined Contribution Plan Disclosure [Line Items]
Cumulative Direct Plans - Net periodic benefit cost	119	83	106
Shared Plans - Multiemployer contribution expense		28	64
Shared Plans - Cost allocation		25	84
Total expense related to all pension and other post retirement benefits	119	136	254
Other Postretirement Benefits [Member]
Defined Contribution Plan Disclosure [Line Items]
Cumulative Direct Plans - Net periodic benefit cost	50	21	(12)
Shared Plans - Multiemployer contribution expense		12	32
Shared Plans - Cost allocation		8	11
Total expense related to all pension and other post retirement benefits	$ 50	$ 41	$ 31

Pension and Other Postretirem93

Pension and Other Postretirement Benefits - Schedule of Obligations and Funded Status (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets at beginning of year	$ 5,436
Fair value of plan assets at end of year	5,320	$ 5,436
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	7,269
Service cost	71	61	$ 51
Interest cost	244	138	89
Settlements	5	16	14
Benefit obligation at end of year	7,639	7,269
Funded status	(2,317)	(1,848)
Less: Amounts attributed to joint venture partners	(37)	(30)
Net funded status	(2,280)	(1,818)
Noncurrent assets	72	43
Current liabilities	(11)	(10)
Noncurrent liabilities	(2,341)	(1,851)
Net amount recognized	(2,280)	(1,818)
Net actuarial loss	3,743	3,254
Prior service cost (benefit)	35	42
Total, before tax effect	3,778	3,296
Less: Amounts attributed to joint venture partners	45	36
Net amount recognized, before tax effect	3,733	3,260
Net actuarial loss (benefit)	676	337
Amortization of accumulated net actuarial (loss) benefit	(187)	(105)
Prior service cost (benefit)	2	2
Amortization of prior service (cost) benefit	(9)	(7)
Total, before tax effect	482	227
Less: Amounts attributed to joint venture partners	9	(3)
Net amount recognized, before tax effect	473	230
Pension Benefits [Member] | Change In Benefit Obligation [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	7,269	2,246
Benefit obligation assumed on August 1, 2016		5,316
Service cost	84	74
Interest cost	250	142
Amendments	2	1
Actuarial losses (gains)	388	(244)
Settlements	(64)	(80)
Benefits paid	(437)	(218)
Foreign currency translation impact	147	32
Benefit obligation at end of year	7,639	7,269	2,246
Pension Benefits [Member] | Change In Plan Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Settlements	(62)	(80)
Benefits paid	(432)	(224)
Foreign currency translation impact	123	22
Fair value of plan assets at beginning of year	5,421	1,891
Fair value of plan assets assumed on August 1, 2016		4,065
Actual return on plan assets	187	(332)
Employer contributions	111	72
Participant contributions	15	18
Administrative expenses	(41)	(11)
Fair value of plan assets at end of year	5,322	5,421	1,891
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	2
Interest cost	38	16	4
Employer contributions	8	6
Funded status	(1,218)	(1,286)
Net funded status	(1,218)	(1,286)
Current liabilities	(118)	(120)
Noncurrent liabilities	(1,100)	(1,166)
Net amount recognized	(1,218)	(1,286)
Net actuarial loss	281	295
Prior service cost (benefit)	(30)	(36)
Total, before tax effect	251	259
Net amount recognized, before tax effect	251	259
Net actuarial loss (benefit)	(1)	(33)
Amortization of accumulated net actuarial (loss) benefit	(13)	(8)
Prior service cost (benefit)		(1)
Amortization of prior service (cost) benefit	6	5
Total, before tax effect	(8)	(37)
Net amount recognized, before tax effect	(8)	(37)
Other Postretirement Benefits [Member] | Change In Benefit Obligation [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation at beginning of year	1,286	82
Benefit obligation assumed on August 1, 2016		1,277
Service cost	5	2
Interest cost	38	16
Actuarial losses (gains)	(1)	(33)
Benefits paid	(116)	(61)
Medicare Part D subsidy receipts	5	3
Foreign currency translation impact	1
Benefit obligation at end of year	$ 1,218	$ 1,286	$ 82

Pension and Other Postretirem94

Pension and Other Postretirement Benefits - Schedule of Obligations and Funded Status (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 5,320	$ 5,436
Domestic Plan [Member] | United States [Member]
Defined Benefit Plan Disclosure [Line Items]
Benefit obligation	5,093	4,977
Fair value of plan assets	3,195	3,504
Funded status	$ (1,898)	$ (1,473)

Pension and Other Postretirem95

Pension and Other Postretirement Benefits - Schedule of Pension Plan Benefit Obligations (Detail) - Pension Benefits [Member] - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Projected benefit obligation	$ 7,639	$ 7,269
Accumulated benefit obligation	7,426	7,075
Projected benefit obligation	7,061	6,699
Fair value of plan assets	4,671	4,807
Accumulated benefit obligation	6,885	6,531
Fair value of plan assets	$ 4,671	$ 4,807

Pension and Other Postretirem96

Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 74	$ 21
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	71	61	$ 51
Interest cost	244	138	89
Expected return on plan assets	(398)	(242)	(121)
Recognized net actuarial loss (benefit)	185	102	42
Amortization of prior service cost (benefit)	9	7	6
Settlements	5	16	14
Curtailments			9
Special termination benefits	3	1	16
Net periodic benefit cost	119	83	106
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	5	2
Interest cost	38	16	4
Recognized net actuarial loss (benefit)	13	8	(3)
Amortization of prior service cost (benefit)	(6)	(5)	(9)
Curtailments			(4)
Net periodic benefit cost	$ 50	$ 21	$ (12)

Pension and Other Postretirem97

Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Cost (Parenthetical) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	$ 74	$ 21
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net periodic benefit cost	50	21	$ (12)
Expense related to pension and other postretirement benefits	$ 8	$ 6

Pension and Other Postretirem98

Pension and Other Postretirement Benefits - Schedule of Amounts Expected to be Recognized in Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition		$ 185	$ 102	$ 42
Prior service cost (benefit) recognition		9	7	6
Pension Benefits [Member] | Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition	$ 223
Prior service cost (benefit) recognition	9
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition		13	8	(3)
Prior service cost (benefit) recognition		$ (6)	$ (5)	$ (9)
Other Postretirement Benefits [Member] | Scenario, Forecast [Member]
Defined Benefit Plan Disclosure [Line Items]
Net actuarial loss recognition	15
Prior service cost (benefit) recognition	$ (6)

Pension and Other Postretirem99

Pension and Other Postretirement Benefits - Schedule of Weighted Average Assumptions Used to Determine Benefit Obligations and Net Periodic Benefit Cost (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]
Rate of compensation increase	3.28%	3.61%
Expected long-term rate of return on plan assets	6.89%
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.68%	4.12%
Rate of compensation increase	3.28%
Discount rate	3.61%	3.45%	4.09%
Expected long-term rate of return on plan assets	7.47%	7.31%	6.91%
Rate of compensation increase	3.61%	3.65%	3.74%
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.54%	3.93%
Discount rate	3.30%	2.90%	4.15%

Pension and Other Postretire100

Pension and Other Postretirement Benefits - Schedule of Assumed Health Care Cost Trend Rates (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Retirement Benefits [Abstract]
Health care cost trend rate assumed for next year	5.50%	5.50%	5.50%
Rate to which the cost trend rate gradually declines	4.50%	4.50%	4.50%
Year that the rate reaches the rate at which it is assumed to remain	2,021	2,020	2,019

Pension and Other Postretire101

Pension and Other Postretirement Benefits - Schedule of One-Percentage Point Change in Assumed Rates of Health Care Cost Trend Rates (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Retirement Benefits [Abstract]
Effect on other postretirement benefit obligations, 1% increase	$ 79
Effect on total of service and interest cost components, 1% increase	3
Effect on other postretirement benefit obligations, 1% decrease	(70)
Effect on total of service and interest cost components, 1% decrease	$ (2)

Pension and Other Postretire102

Pension and Other Postretirement Benefits - Schedule of Pension and Postretirement Plans Investment Policy and Weighted Average Asset Allocations (Detail)	Dec. 31, 2017	Dec. 31, 2016
Equity Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	20.00%
Equity Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations		55.00%
Fixed Income Securities [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	25.00%
Fixed Income Securities [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations		55.00%
Other Investments [Member] | Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	15.00%
Other Investments [Member] | Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations		55.00%
Plan Assets [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	100.00%	100.00%
Plan Assets [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	40.00%	37.00%
Plan Assets [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	35.00%	36.00%
Plan Assets [Member] | Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Defined benefit plan, asset allocations	25.00%	27.00%

Pension and Other Postretire103

Pension and Other Postretirement Benefits - Schedule of Fair Value of Pension Plan Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	$ 5,320	$ 5,436
Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	2,153	1,987
Equity Securities [Member] | Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,775	1,292
Equity Securities [Member] | Hedge Funds, Equity Long (Short) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	152	449
Equity Securities [Member] | Private Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	226	246
Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,848	1,954
Fixed Income Securities [Member] | Intermediate and Long Duration Government Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	737	528
Fixed Income Securities [Member] | Cash and Cash Equivalent Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,055	1,365
Fixed Income Securities [Member] | Other Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	56	61
Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,319	1,495
Other Investments [Member] | Other Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	132	210
Other Investments [Member] | Real Estate Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	606	419
Other Investments [Member] | Discretionary and Systematic Macro Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	581	866
Level 1 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,555	1,654
Level 1 [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	906	520
Level 1 [Member] | Equity Securities [Member] | Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	906	520
Level 1 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	408	975
Level 1 [Member] | Fixed Income Securities [Member] | Intermediate and Long Duration Government Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	95	78
Level 1 [Member] | Fixed Income Securities [Member] | Cash and Cash Equivalent Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	313	897
Level 1 [Member] | Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	241	159
Level 1 [Member] | Other Investments [Member] | Other Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets		71
Level 1 [Member] | Other Investments [Member] | Real Estate Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	241	88
Level 2 [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	434	344
Level 2 [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	434	344
Level 2 [Member] | Fixed Income Securities [Member] | Intermediate and Long Duration Government Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	378	283
Level 2 [Member] | Fixed Income Securities [Member] | Other Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	56	61
Net Asset Value [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	3,331	3,438
Net Asset Value [Member] | Equity Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,247	1,467
Net Asset Value [Member] | Equity Securities [Member] | Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	869	772
Net Asset Value [Member] | Equity Securities [Member] | Hedge Funds, Equity Long (Short) [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	152	449
Net Asset Value [Member] | Equity Securities [Member] | Private Equity Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	226	246
Net Asset Value [Member] | Fixed Income Securities [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,006	635
Net Asset Value [Member] | Fixed Income Securities [Member] | Intermediate and Long Duration Government Credit [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	264	167
Net Asset Value [Member] | Fixed Income Securities [Member] | Cash and Cash Equivalent Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	742	468
Net Asset Value [Member] | Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	1,078	1,336
Net Asset Value [Member] | Other Investments [Member] | Other Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	132	139
Net Asset Value [Member] | Other Investments [Member] | Real Estate Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	365	331
Net Asset Value [Member] | Other Investments [Member] | Discretionary and Systematic Macro Hedge Funds [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of pension and other postretirement plans' assets	$ 581	$ 866

Pension and Other Postretire104

Pension and Other Postretirement Benefits - Schedule of Fair Value of Pension Plan Assets (Parenthetical) (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Sale Of Subsidiary [Abstract]
Net receivables which represents assets related to divested businesses to be transferred to the buyers	$ 2	$ 15

Pension and Other Postretire105

Pension and Other Postretirement Benefits - Schedule of Benefit Payments Expected to be Paid and Expected Medicare Part D Subsidy Receipts (Detail) $ in Millions	Dec. 31, 2017USD ($)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2,018	$ 490
2,019	485
2,020	490
2,021	490
2,022	490
2023 through 2027	2,400
Total benefit payments	4,845
Gross Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2,018	125
2,019	125
2,020	120
2,021	120
2,022	115
2023 through 2027	425
Total benefit payments	1,030
Medicare Part D Subsidy Receipts [Member]
Defined Benefit Plan Disclosure [Line Items]
2,018	10
2,019	10
2,020	5
2,021	5
2,022	5
2023 through 2027	25
Total benefit payments	60
Net Other Post Retirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
2,018	115
2,019	115
2,020	115
2,021	115
2,022	110
2023 through 2027	400
Total benefit payments	$ 970

Derivatives and Other Financ106

Derivatives and Other Financial Instruments - Additional Information (Detail) kT in Thousands	1 Months Ended		12 Months Ended
	Aug. 31, 2016USD ($)	Apr. 30, 2016Refinery	Dec. 31, 2017USD ($)MWhDerivativeRefinerySmelterkT	Dec. 31, 2016USD ($)MWhkT	Dec. 31, 2015USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative contracts recorded as assets			$ 197,000,000	$ 514,000,000
Fair value of derivative contracts recorded as liabilities			$ 1,173,000,000	267,000,000
Number of refineries | Refinery		3
Other derivative contracts estimated term of quoted market prices, in years			10 years
Decrease in unrealized gain			$ (24,000,000)	(9,000,000)	$ (26,000,000)
Recognized an unrealized gain (loss)			$ 25,000,000
Electricity purchases			In addition, in January 2017, Alcoa Corporation entered into a new financial contract that hedges the anticipated power requirements at this smelter for the period from August 2017 through July 2021 (see D11 above). At December 31, 2017, this financial contract hedges forecasted electricity purchases of 8,805,456 megawatt hours.
Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative contracts recorded as assets			$ 44,000,000	5,000,000
Fair value of derivative contracts recorded as liabilities			117,000,000	2,000,000
Other Comprehensive Loss [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) recognized			88,000,000
Other Income [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Decrease in unrealized gain				7,000,000
Cost of Goods Sold [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Decrease in unrealized gain				6,000,000
Realized gain (loss)			96,000,000
Other Expenses (Income) [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) recognized			(22,000,000)		4,000,000
Other Expenses (Income) [Member] | Maximum [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) recognized				(1,000,000)
Realized gain (loss)			(1,000,000)
Cash Flow Hedging [Member] | Other Comprehensive Loss [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Unrealized gain (loss) recognized			$ (92,000,000)	2,000,000	(13,000,000)
London Metal Exchange [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative			2
LME Plus Midwest Premium [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative			3
Derivatives Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative contracts recorded as assets			$ 197,000,000	497,000,000
Fair value of derivative contracts recorded as liabilities			$ 1,112,000,000	204,000,000
Number of derivative instruments | Derivative			1
Derivatives Not Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value of derivative contracts recorded as assets				17,000,000
Fair value of derivative contracts recorded as liabilities			$ 61,000,000	63,000,000
Number of derivative instruments | Derivative			1
Derivatives Not Designated as Hedging Instruments [Member] | Other Expenses (Income), Net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net gain (loss) of derivative instruments			$ (22,000,000)	(17,000,000)	(25,000,000)
Embedded Aluminum Derivative [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Recognized an unrealized gain (loss)			(1,521,000,000)	(615,000,000)	1,155,000,000
Realized gain (loss) on derivatives			$ (113,000,000)	$ (7,000,000)	(21,000,000)
Embedded Aluminum Derivative [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of smelters | Smelter			9
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Aluminum forecast sales | kT			2,859	3,127
Embedded Aluminum Derivative [Member] | Derivatives Designated as Hedging Instruments [Member] | Cash Flow Hedging [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Amount of gain (loss) expected to be recognized into earnings over the next 12 months				$ (120,000,000)
Derivative instruments ineffectiveness			$ 0	0	0
Embedded Aluminum Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net gain (loss) of derivative instruments			$ (18,000,000)	(15,000,000)	(8,000,000)
Embedded Aluminum Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of derivative instruments | Derivative			2
Financial Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in derivative asset				(84,000,000)
Recognized an unrealized gain (loss)				96,000,000	(2,000,000)
Financial Contracts [Member] | Derivatives Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative instruments ineffectiveness				$ 3,000,000	(3,000,000)
Forecasted energy purchases in megawatt hours | MWh			8,805,456	1,969,544
Energy Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Decrease in unrealized gain	$ 84,000,000
Energy Contracts [Member] | Other Expenses (Income), Net [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase (decrease) in derivative asset	$ 84,000,000
Energy Contracts [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Number of refineries | Refinery			3
Energy Contracts [Member] | Derivatives Not Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net gain (loss) of derivative instruments			$ (7,000,000)	$ 3,000,000	0
Embedded Credit Derivative [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative contract period			10 years
Embedded Credit Derivative [Member] | Negotiated multiplier [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Derivative contract period			20 years
Embedded Credit Derivative [Member] | Derivatives Not Designated as Hedging Instruments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Net gain (loss) of derivative instruments			$ 3,000,000	$ (5,000,000)	$ (17,000,000)

Derivatives and Other Financ107

Derivatives and Other Financial Instruments - Schedule of Quantitative Information for Level 3 Derivative Contracts (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)MWh$ / Metric_Ton$ / lb$ / MWh$ / Barrels	Dec. 31, 2016USD ($)
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value | $	$ 197	$ 514
Interrelationship of LME Price to Overall Energy Price [Member] | Level 3 [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 24
Embedded Aluminum Derivative [Member] | Level 3 [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Maturity year of future aluminum price	2,019
Embedded Aluminum Derivative [Member] | Interrelationship of LME Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum One [Member] | Level 3 [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Maturity year of future aluminum price | MWh	4,000,000
Derivative Liabilities, Fair value | $	$ 403
Embedded Aluminum Derivative [Member] | Interrelationship of LME Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum One [Member] | Level 3 [Member] | Minimum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	2,258
Maturity year of future aluminum price	2,018
Embedded Aluminum Derivative [Member] | Interrelationship of LME Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum One [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	2,651
Maturity year of future aluminum price	2,027
Embedded Aluminum Derivative [Member] | Interrelationship of LME Price to Amount of Megawatt Hours of Energy Needed to Produce Forecasted Metric Tons of Aluminum [Member] | Level 3 [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 34
Embedded Aluminum Derivative [Member] | Interrelationship of LME Price to Overall Energy Price [Member] | Level 3 [Member] | Average Price [Member] | Minimum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Maturity year of future aluminum price | MWh	115,000
Expected future aluminum prices	2,019
Embedded Aluminum Derivative [Member] | Interrelationship of LME Price to Overall Energy Price [Member] | Level 3 [Member] | Average Price [Member] | Maximum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Maturity year of future aluminum price | MWh	2
Expected future aluminum prices	2,342
Embedded Aluminum Derivative [Member] | Price of Aluminum beyond Forward Curve [Member] | Level 3 [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 685
Expected future aluminum prices	2,258
Maturity year of future aluminum price	2,018
Embedded Aluminum Derivative [Member] | Price of Aluminum beyond Forward Curve [Member] | Level 3 [Member] | Minimum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Maturity year of future aluminum price	2,018
Midwest Premium | $ / lb	2,317
Embedded Aluminum Derivative [Member] | Price of Aluminum beyond Forward Curve [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Midwest Premium | $ / lb	0.095
Maturity year of future aluminum price	2,019
Embedded Aluminum Derivative [Member] | Price of Aluminum beyond Forward Curve [Member] | Level 3 [Member] | Average Price [Member] | Minimum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	2,679
Maturity year of future aluminum price	2,028
Embedded Aluminum Derivative [Member] | Price of Aluminum beyond Forward Curve [Member] | Level 3 [Member] | Midwest Premium [Member] | Minimum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Midwest Premium | $ / lb	0.095
Maturity year of future aluminum price	2,018
Embedded Aluminum Derivative [Member] | Interrelationship of Future Aluminum and Oil Prices [Member] | Level 3 [Member] | Average Price [Member] | Minimum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	2,258
Maturity month and year of future aluminum price	2018-01
Expected future oil prices | $ / Barrels	67
Maturity month and year of future oil price	2018-01
Embedded Aluminum Derivative [Member] | Interrelationship of Future Aluminum and Oil Prices [Member] | Level 3 [Member] | Average Price [Member] | Maximum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	2,299
Maturity month and year of future aluminum price	2018-10
Expected future oil prices | $ / MWh	64
Maturity month and year of future oil price	2018-10
Embedded Aluminum Derivative [Member] | Two Contracts [Member] | Price of Aluminum beyond Forward Curve [Member] | Level 3 [Member] | Average Price [Member] | Maximum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	2,759
Maturity year of future aluminum price	2,029
Embedded Aluminum Derivative [Member] | Two Contracts [Member] | Price of Aluminum beyond Forward Curve [Member] | Level 3 [Member] | Midwest Premium [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Midwest Premium | $ / lb	0.115
Embedded Aluminum Derivative [Member] | Two Contracts [Member] | Price of Aluminum beyond Forward Curve [Member] | Level 3 [Member] | Midwest Premium [Member] | Maximum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Maturity year of future aluminum price	2,029
Embedded Aluminum Derivative [Member] | One Contract [Member] | Price of Aluminum beyond Forward Curve [Member] | Level 3 [Member] | Average Price [Member] | Minimum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Expected future aluminum prices	3,055
Maturity year of future aluminum price	2,036
Embedded Aluminum Derivative [Member] | One Contract [Member] | Price of Aluminum beyond Forward Curve [Member] | Level 3 [Member] | Midwest Premium [Member] | Maximum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Maturity year of future aluminum price	2,036
Financial Contracts [Member] | Interrelationship of Forward Energy Price and the Consumer Price Index and Price of Electricity Beyond Forward Curve [Member] | Level 3 [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Assets, Fair value | $	$ 197
Financial Contracts [Member] | Interrelationship of Forward Energy Price and the Consumer Price Index and Price of Electricity Beyond Forward Curve [Member] | Level 3 [Member] | Minimum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price of electricity beyond forward curve | $ / MWh	83.69
Maturity date of electricity beyond forward curve	2,018
Financial Contracts [Member] | Interrelationship of Forward Energy Price and the Consumer Price Index and Price of Electricity Beyond Forward Curve [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Price of electricity beyond forward curve | $ / MWh	53.60
Maturity date of electricity beyond forward curve	2,021
Embedded Credit Derivative [Member] | Estimated Difference In Credit Spread Of Each Of Alcoa Corporation And Counterparty, And Negotiated Multiplier [Member] | Level 3 [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Derivative Liabilities, Fair value | $	$ 27
Percentage of credit spread	3.38%
Embedded Credit Derivative [Member] | Estimated Difference In Credit Spread Of Each Of Alcoa Corporation And Counterparty, And Negotiated Multiplier [Member] | Level 3 [Member] | Negotiated multiplier [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Multiplier percentage used to calculate credit spread	1.87%
Embedded Credit Derivative [Member] | Estimated Difference In Credit Spread Of Each Of Alcoa Corporation And Counterparty, And Negotiated Multiplier [Member] | Level 3 [Member] | Counterparty [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage of credit spread	1.51%
Embedded Credit Derivative [Member] | Estimated Difference In Credit Spread Of Each Of Alcoa Corporation And Counterparty, And Negotiated Multiplier [Member] | Level 3 [Member] | Alcoa Corporation [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage of credit spread	2.48%
Embedded Credit Derivative [Member] | Estimated Difference In Credit Spread Of Each Of Alcoa Corporation And Counterparty, And Negotiated Multiplier [Member] | Level 3 [Member] | Maximum [Member]
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Percentage of credit spread	3.49%

Derivatives and Other Financ108

Derivatives and Other Financial Instruments - Schedule of Fair Values of Level 3 Derivative Instruments Recorded as Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	$ 197	$ 514
Fair value liability derivatives	1,173	267
Derivatives Designated as Hedging Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	197	497
Fair value liability derivatives	1,112	204
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives		29
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Financial Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	96
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives		468
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Financial Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives	101
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value liability derivatives	120	17
Derivatives Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value liability derivatives	992	187
Derivatives Not Designated as Hedging Instruments [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives		17
Fair value liability derivatives	61	63
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Financial Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset derivatives		17
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value liability derivatives	28	10
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Current [Member] | Embedded Credit Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value liability derivatives	4	5
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value liability derivatives	6	18
Derivatives Not Designated as Hedging Instruments [Member] | Fair Value of Derivative Contracts - Noncurrent [Member] | Embedded Credit Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value liability derivatives	$ 23	$ 30

Derivatives and Other Financ109

Derivatives and Other Financial Instruments - Schedule of Net Fair Values of Level 3 Derivative Instruments and Effect of Hypothetical Change (Increase or Decrease of 10%) in Market Prices or Rates (Detail) $ in Millions	Dec. 31, 2017USD ($)
Embedded Aluminum Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset/(liability)	$ (1,146)
Index change of + / - 10%	489
Embedded Credit Derivative [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset/(liability)	(27)
Index change of + / - 10%	3
Financial Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Fair value asset/(liability)	197
Index change of + / - 10%	$ 55

Derivatives and Other Financ110

Derivatives and Other Financial Instruments - Schedule of Reconciliation of Activity for Derivative Contracts (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Energy Contracts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets, Beginning balance	$ 17	$ 2
Fair value measurement, Assets, Other comprehensive income (loss)		95
Fair value measurement, Assets, Transfers into and/or out of Level 3		0
Fair value measurement, Assets, Ending balance		17
Fair value measurement, Assets, Sales		0
Fair value measurement, Assets, Cost of goods sold		0
Energy Contracts [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets		(80)
Embedded Aluminum Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets, Beginning balance	497	1,135
Fair value measurement, Assets, Other comprehensive income (loss)	(499)	(568)
Fair value measurement, Assets, Transfers into and/or out of Level 3	0	0
Fair value measurement, Assets, Other	(2)	40
Fair value measurement, Assets, Ending balance		497
Fair value measurement, Assets, Sales	0	0
Fair value measurement, Assets, Cost of goods sold	0	0
Fair value measurement, Liabilities, Beginning balance	232	169
Fair value measurement, Liabilities, Other comprehensive income (loss)	1,022	47
Fair value measurement, Liabilities, Purchases, sales, issuances, and settlements		32
Fair value measurement, Liabilities,Transfers into and/or out of Level 3	0	0
Fair value measurement, Liabilities, Other	(16)	(6)
Fair value measurement, Liabilities, Ending balance	1,146	232
Fair value measurement, Liabilities, Sales	0	0
Fair value measurement, Liabilities, Cost of goods sold	0	0
Fair value measurement, Liabilities, Other income, net	18
Embedded Aluminum Derivative [Member] | Sales [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	3	(5)
Fair value measurement, Liabilities	(110)	(12)
Embedded Aluminum Derivative [Member] | Other Expense [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	1
Fair value measurement, Liabilities	18
Embedded Aluminum Derivative [Member] | Cost of Goods Sold [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets		(92)
Embedded Aluminum Derivative [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	1	(13)
Fair value measurement, Liabilities		2
Embedded Credit Derivative [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities, Beginning balance	35	35
Fair value measurement, Liabilities, Cost of goods sold	(5)	(5)
Fair value measurement, Liabilities,Transfers into and/or out of Level 3	0	0
Fair value measurement, Liabilities, Ending balance	27	35
Fair value measurement, Liabilities, Sales	0	0
Fair value measurement, Liabilities, Cost of goods sold	0	0
Fair value measurement, Liabilities, Other income, net	(3)
Embedded Credit Derivative [Member] | Other Expense [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities	(3)
Embedded Credit Derivative [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Liabilities		5
Financial Contracts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets, Beginning balance	17
Fair value measurement, Assets, Other comprehensive income (loss)	88
Fair value measurement, Assets, Purchases, sales, issuances, and settlements	119
Fair value measurement, Assets, Transfers into and/or out of Level 3	0
Fair value measurement, Assets, Other	11
Fair value measurement, Assets, Ending balance	197	17
Fair value measurement, Assets, Sales	0
Fair value measurement, Assets, Cost of goods sold	0
Fair value measurement, Liabilities, Beginning balance		4
Fair value measurement, Liabilities, Other comprehensive income (loss)		(1)
Fair value measurement, Liabilities,Transfers into and/or out of Level 3		0
Fair value measurement, Liabilities, Other		(1)
Fair value measurement, Liabilities, Sales		0
Fair value measurement, Liabilities, Cost of goods sold		0
Financial Contracts [Member] | Other Expense [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	(7)
Financial Contracts [Member] | Cost of Goods Sold [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	(31)
Financial Contracts [Member] | Other Income [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Fair value measurement, Assets	$ (7)
Fair value measurement, Liabilities		$ (2)

Derivatives and Other Financ111

Derivatives and Other Financial Instruments - Schedule of Reconciliation of Activity for Derivative Contracts (Parenthetical) (Detail) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Aug. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Decrease in unrealized gain		$ (24)	$ (9)	$ (26)
Energy Contracts [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Decrease in unrealized gain	$ 84
Energy Contracts [Member] | Other Expenses (Income), Net [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Decreased derivative asset	$ 84

Derivatives and Other Financ112

Derivatives and Other Financial Instruments - Schedule of Carrying Values and Fair Values of Other Financial Instruments (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Carrying Value [Member]
Derivative [Line Items]
Cash and cash equivalents	$ 1,358	$ 853
Restricted cash	7	6
Long-term debt due within one year	16	21
Long-term debt, less amount due within one year	1,388	1,424
Fair Value [Member]
Derivative [Line Items]
Cash and cash equivalents	1,358	853
Restricted cash	7	6
Long-term debt due within one year	16	21
Long-term debt, less amount due within one year	$ 1,555	$ 1,573

Income Taxes - Components of Lo

Income Taxes - Components of Loss from Continuing Operations Before Income Taxes (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Domestic	$ (712)	$ (688)	$ (1,053)
Foreign	1,871	526	716
Income (Loss) before income taxes	$ 1,159	$ (162)	$ (337)

Income Taxes - Schedule of Prov

Income Taxes - Schedule of Provision for Income Taxes on Income from Continuing Operations (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Current:
Federal	$ 3	$ 9	$ 3
Foreign	421	221	313
State and local	0	0	0
Current provision for income taxes, total	424	230	316
Deferred:
Federal	24		(85)
Foreign	152	(46)	171
State and local	0	0	0
Deferred provision for income taxes, total	176	(46)	86
Provision for income taxes	$ 600	$ 184	$ 402

Income Taxes - Reconciliation o

Income Taxes - Reconciliation of U.S. Federal Statutory Rate to Alcoa's Effective Tax Rate (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
U.S. federal statutory rate	21.00%	21.00%	35.00%	35.00%	35.00%
Changes in valuation allowances			25.80%	(1.90%)	(62.60%)
Taxes on foreign operations-rate differential			(10.80%)	44.30%	14.20%
Impact of U.S. Tax Cuts and Jobs Act of 2017			1.90%
Noncontrolling interest			1.40%	(7.30%)	(8.50%)
Other taxes related to foreign operations			1.30%	(19.50%)	(20.90%)
Tax holidays			0.40%	11.20%	6.20%
Losses and credits with no tax benefit			(0.20%)	(163.20%)	(82.00%)
Statutory tax rate and law changes			0.10%	(0.60%)	(0.30%)
Nondeductible costs related to the Separation Transaction				(9.60%)
Impact of capitalization of intercompany debt					3.30%
Other			(3.10%)	(2.00%)	(3.70%)
Effective tax rate			51.80%	(113.60%)	(119.30%)
Change in valuation allowance due to change in tax rate			$ 20
Benefit from tax holiday rates			$ 20
Income tax benefit per share			$ 0.11

Income Taxes - Additional Infor

Income Taxes - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Taxes [Line Items]
Corporate income tax rate	21.00%	21.00%	35.00%	35.00%	35.00%
Net deferred tax assets	$ 517		$ 517
Deferred tax assets	3,112		3,112	$ 3,088
Deferred tax liabilities	668		668
Deferred tax assets, valuation allowance	1,927		1,927	1,755	$ 712	$ 486
Valuation allowance	0		0	$ 0
Foreign undistributed net earnings for which no deferred taxes have been provided	940		940
Earnings generated during 2017 deemed to be permanently reinvested	150		$ 150
Percentage of the effect of unrecognized tax benefit, if recorded			1.00%	10.00%	4.00%
Interest and penalties recognized			$ 1	$ 1	$ 7
Income related to accrued interest and penalties			6	2	$ 1
Amount accrued for payment of interest and penalties	2		$ 2	$ 6
Income tax rate on global intangible low tax income		10.50%
Net operating losses carry forward period		0 years	20 years
Net operating losses carryback period		0 years	2 years
Percentage of taxable income related to net operating losses carry forward and carryback period		80.00%	100.00%
Charge resulting from preliminary analysis of provisions of TCJA			$ 22
Change in deferred tax assets related to TCJA			(506)
Change in valuation allowance related to TCJA			(433)
Change in deferred tax liabilities related to TCJA			$ (51)
Threshold percentage related to Tax Cuts and Jobs Act of 2017			3.00%
Alcoa World Alumina Brasil [Member]
Income Taxes [Line Items]
Decrease in deferred tax asset due to tax holiday			$ 26
Decrease in discrete income tax charge due tax holiday			26
Decrease in deferred tax asset due to tax holiday after noncontrolling interest			15
Decrease in discrete income tax charge due to tax holiday after noncontrolling interest			$ 15
Corporate income tax rate			15.25%
Tax holiday period			10 years
Iceland [Member]
Income Taxes [Line Items]
Deferred tax assets, valuation allowance	94		$ 94
Discrete income tax charge charge			60
Tax charge to Accumulated other comprehensive loss	34		34
Valuation allowance	56		56
Iceland And Suriname [Member]
Income Taxes [Line Items]
Discrete income tax charge charge			141
Suriname [Member]
Income Taxes [Line Items]
Deferred tax assets, valuation allowance	85		$ 85
Minimum [Member]
Income Taxes [Line Items]
One-time deemed repatriation of foreign undistributed net earnings, percentage			8.00%
Tax rate on base erosion payments		5.00%
Percentage of deductible expenses			3.00%
Maximum [Member]
Income Taxes [Line Items]
One-time deemed repatriation of foreign undistributed net earnings, percentage			15.50%
Tax rate on base erosion payments		10.00%
Secretariat of the Federal Revenue Bureau of Brazil [Member]
Income Taxes [Line Items]
Tax credit carryforward, limitations on use			30.00%
Tax Authority, Spain [Member]
Income Taxes [Line Items]
Tax credit carryforward, limitations on use			25.00%
Foreign [Member]
Income Taxes [Line Items]
Net deferred tax assets	511		$ 511
Deferred tax assets	1,948		1,948
Deferred tax liabilities	560		560
Deferred tax assets, valuation allowance	877		877
Foreign [Member] | Alcoa Aluminio [Member]
Income Taxes [Line Items]
Deferred tax assets	252		252
Foreign [Member] | Alcoa World Alumina Brasil [Member]
Income Taxes [Line Items]
Deferred tax assets	153		153
Foreign [Member] | Alumina Espanola, S.A. [Member]
Income Taxes [Line Items]
Deferred tax assets	112		112
Foreign [Member] | Other Foreign Countries [Member]
Income Taxes [Line Items]
Deferred tax liabilities	$ 6		$ 6
State and Local Jurisdiction [Member] | Earliest Tax Year [Member]
Income Taxes [Line Items]
Income tax return, year under examination			2,006
State and Local Jurisdiction [Member] | Latest Tax Year [Member]
Income Taxes [Line Items]
Income tax return, year under examination			2,015

Income Taxes - Schedule of Comp

Income Taxes - Schedule of Components of Net Deferred Tax Assets and Liabilities (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Abstract]
Tax loss carryforwards	$ 1,185	$ 1,064
Deferred tax assets, Employee benefits	949	1,240
Deferred tax assets, Derivatives and hedging activities	287
Deferred tax assets, Loss provisions	246	313
Deferred tax assets, Tax credit carryforwards	193	23
Deferred tax assets, Depreciation	141	187
Deferred tax assets, Deferred income/expense	11	28
Deferred tax assets, Other	100	233
Deferred tax assets, Gross	3,112	3,088
Deferred tax assets, Valuation allowance	(1,927)	(1,755)	$ (712)	$ (486)
Deferred tax assets, net	1,185	1,333
Deferred tax liabilities, Tax loss carryforwards	0	0
Deferred tax liabilities, Employee benefits	0	0
Deferred tax liabilities, Derivatives and hedging activities	70	124
Deferred tax liabilities, Loss provisions	0	0
Deferred tax liabilities, Tax credit carryforwards	0	0
Deferred tax liabilities, Depreciation	432	499
Deferred tax liabilities, Deferred income/expense	109	136
Deferred tax liabilities, Other	57	125
Deferred tax liabilities, Gross	668	884
Deferred tax liabilities, Valuation allowance	0	0
Deferred tax liabilities, Net	$ 668	$ 884

Income Taxes - Schedule of Expi

Income Taxes - Schedule of Expiration Periods of Deferred Tax Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Tax Credit Carryforward [Line Items]
Tax loss carryforwards	$ 1,185
Tax credit carryforwards	193	$ 23
Other	1,734
Valuation allowance	(1,927)	(1,755)	$ (712)	$ (486)
Deferred tax assets, net	1,185	$ 1,333
Expires Within 10 Years [Member]
Tax Credit Carryforward [Line Items]
Tax loss carryforwards	330
Tax credit carryforwards	193
Valuation allowance	(523)
Expires Within 11-20 Years [Member]
Tax Credit Carryforward [Line Items]
Tax loss carryforwards	250
Valuation allowance	(152)
Deferred tax assets, net	98
No Expiration [Member]
Tax Credit Carryforward [Line Items]
Tax loss carryforwards	605
Other	297
Valuation allowance	(315)
Deferred tax assets, net	587
Other [Member]
Tax Credit Carryforward [Line Items]
Other	1,437
Valuation allowance	(937)
Deferred tax assets, net	$ 500

Income Taxes - Composition of N

Income Taxes - Composition of Net Deferred Tax Asset by Jurisdiction (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Disclosure [Line Items]
Deferred tax assets	$ 3,112	$ 3,088
Valuation allowance	(1,927)	$ (1,755)	$ (712)	$ (486)
Deferred tax liabilities	(668)
Total deferred tax asset	517
Domestic [Member]
Income Tax Disclosure [Line Items]
Deferred tax assets	1,164
Valuation allowance	(1,050)
Deferred tax liabilities	(108)
Total deferred tax asset	6
Foreign [Member]
Income Tax Disclosure [Line Items]
Deferred tax assets	1,948
Valuation allowance	(877)
Deferred tax liabilities	(560)
Total deferred tax asset	$ 511

Income Taxes - Schedule of Chan

Income Taxes - Schedule of Changes in Valuation Allowance (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ (1,755)	$ (712)	$ (486)
Establishment of new allowances	(94)		(141)
Net change to existing allowances	(33)	(1,056)	(148)
U.S. state tax apportionment and tax rate changes			30
Foreign currency translation	(45)	13	33
Balance at end of year	$ (1,927)	$ (1,755)	$ (712)

Income Taxes - Reconciliatio121

Income Taxes - Reconciliation of Unrecognized Tax Benefits (Excluding Interest and Penalties) (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance at beginning of year	$ 23	$ 22	$ 25
Additions for tax positions of the current year	1	3	2
Additions for tax positions of prior years		1	1
Reductions for tax positions of prior years	(5)	(2)
Settlements with tax authorities	(6)	(2)	(2)
Expiration of the statute of limitations	(3)
Foreign currency translation		1	(4)
Balance at end of year	$ 10	$ 23	$ 22

Asset Retirement Obligations -

Asset Retirement Obligations - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017USD ($)Structure	Dec. 31, 2016USD ($)
Asset Retirement Obligation Disclosure [Abstract]
Estimated CARO ranges per structure, minimum	$ 3,000,000
Estimated CARO ranges per structure, maximum	$ 28,000,000
Number of structures required to demolish | Structure	24
Current liability	$ 108,000,000	$ 104,000,000

Asset Retirement Obligations123

Asset Retirement Obligations - Schedule of Carrying Value of Recorded AROs by Major Category (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asset Retirement Obligation Disclosure [Abstract]
Mine reclamation	$ 221	$ 199
Closure of bauxite residue areas	238	219
Spent pot lining disposal	125	135
Demolition	113	121
Landfill closure	27	30
Other	1	4
Asset retirement obligation, total	$ 725	$ 708	$ 635

Asset Retirement Obligations124

Asset Retirement Obligations - Schedule of Changes in Carrying Value of Recorded AROs (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Asset Retirement Obligation Disclosure [Abstract]
Balance at beginning of year	$ 708	$ 635
Accretion expense	17	19
Payments	(69)	(47)
Liabilities incurred	43	117
Foreign currency translation and other	26	(16)
Balance at end of year	$ 725	$ 708

Contingencies and Commitments -

Contingencies and Commitments - Additional Information (Detail)	Jan. 31, 2018EUR (€)	Jan. 31, 2018USD ($)	Nov. 19, 2009EUR (€)Plant	Jun. 30, 2012EUR (€)Installment	Jun. 30, 2012USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2017USD ($)	Dec. 31, 2016USD ($)	Dec. 31, 2015EUR (€)	Dec. 31, 2015USD ($)	Dec. 31, 2013EUR (€)	Dec. 31, 2013USD ($)	Feb. 21, 2013EUR (€)	Feb. 21, 2013USD ($)	Dec. 31, 2012kt	Jun. 30, 2012USD ($)Installment	Mar. 26, 2012EUR (€)	Mar. 26, 2012USD ($)	Sep. 30, 2010kt	Nov. 19, 2009USD ($)
Loss Contingencies [Line Items]
Management estimate for maximum exposure from class action																		€ 85,000,000	$ 110,000,000
Partial reserve										€ 34,000,000	$ 37,000,000
Recovery amount				€ 303,000,000	$ 375,000,000
Reduction in recovery amount				53,000,000													$ 65,000,000
Payments to Italian Government				€ 250,000,000	$ 310,000,000
Payments to Italian Government, installments amount												€ 50,000,000	$ 69,000,000
Number of installments, litigation payment | Installment				5													5
Restructuring and other charges | $						$ 297,000,000	$ 209,000,000	$ 309,000,000	$ 318,000,000		$ 983,000,000
Amount of curtailment, metric-tons-per-year | kt																				44,000
Subsequent Event [Member]
Loss Contingencies [Line Items]
Payment to CCSE and Energy authority, litigation settlement	€ 15,000,000	$ 18,000,000
Reduction in restructuring and other charges reserve	€ 19,000,000	$ 22,000,000
ITALY
Loss Contingencies [Line Items]
Number of years operating under a power supply structure										10 years	10 years
Charge related to European Commission announcement			€ 173,000,000																		$ 250,000,000
Number of smelters | Plant			2
Write-off of receivable			€ 14,000,000																		20,000,000
Establishment reserve			€ 159,000,000																		$ 230,000,000
IT [Member]
Loss Contingencies [Line Items]
Noncurrent asset excess of total assets										€ 91,000,000	$ 100,000,000
Amount owned by owed by the Italian Government to Alcoa										53,000,000	58,000,000
Interest previously paid,write-off										6,000,000	6,000,000
Restructuring and other charges										€ 150,000,000	$ 164,000,000
Portovesme [Member]
Loss Contingencies [Line Items]
Amount of curtailment, metric-tons-per-year | kt																150,000
Alcoa Trasformazioni [Member]
Loss Contingencies [Line Items]
Management estimate for maximum exposure from class action														€ 76,000,000	$ 97,000,000
Minimum [Member]
Loss Contingencies [Line Items]
Provision for contract losses | $															0
Maximum [Member]
Loss Contingencies [Line Items]
Provision for contract losses														€ 76,000,000	$ 97,000,000

Contingencies and Commitment126

Contingencies and Commitments - Additional Information - 1 (Detail) - USD ($) $ in Millions	1 Months Ended		6 Months Ended	12 Months Ended
	Sep. 30, 2015	Jun. 30, 2015	Jun. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2012	Dec. 31, 2009
Loss Contingencies [Line Items]
Remediation reserve balance				$ 294	$ 324
Remediation reserve balance, classified as a current liability				36	60
Increase in remediation reserve	$ 1
Payments related to remediation expenses applied against the reserve				48	32	$ 24
Increase (Decrease) in reserves due to effects of foreign currency translation				11	5	(13)
Reclassification of amounts included in asset retirement obligations				5
Reclassification of obligation transferred on separation				17	60
Reclassification of amounts included in other reserves						17
Rockdale Smelter Curtailment [Member]
Loss Contingencies [Line Items]
Increase in remediation reserve				1
Remediation reserve adjustment				8
Baie Comeau and Mosjen [Member]
Loss Contingencies [Line Items]
Remediation reserve adjustment				6
Warrick Smelter, Wenatchee Smelter and Point Comfort [Member]
Loss Contingencies [Line Items]
Remediation reserve adjustment				4
Other Sites [Member]
Loss Contingencies [Line Items]
Remediation reserve adjustment				3	13
Suriname Refinery and Permanent Closure [Member]
Loss Contingencies [Line Items]
Increase in remediation reserve					39
Remediation reserve adjustment					26
Massena East Smelter [Member]
Loss Contingencies [Line Items]
Increase in remediation reserve						107
Remediation reserve adjustment						52
Pocos De Caldas Smelter And The Anglesea Power Station [Member]
Loss Contingencies [Line Items]
Remediation reserve adjustment						29
Mosjoen [Member]
Loss Contingencies [Line Items]
Remediation reserve balance				2	8
Increase in remediation reserve		$ 11	$ (2)			12		$ 20
Portovesme [Member]
Loss Contingencies [Line Items]
Increase in remediation reserve							$ 3		$ 3
Remediation reserve adjustment						7
Sherwin, TX Site [Member]
Loss Contingencies [Line Items]
Remediation reserve balance				29	30
Several Sites [Member]
Loss Contingencies [Line Items]
Remediation reserve balance				150	141
Alcoa Corporation [Member] | Warrick Smelter, Wenatchee Smelter and Point Comfort [Member]
Loss Contingencies [Line Items]
Remediation reserve adjustment				$ 4	$ 26	$ 86

Contingencies and Commitment127

Contingencies and Commitments - Additional Information - 2 (Detail) € in Millions, AUD in Millions

Dec. 31, 2017EUR (€)

Apr. 08, 2015USD ($)InstallmentRefinery

Feb. 27, 2014EUR (€)

Feb. 27, 2014USD ($)

Apr. 08, 2013USD ($)

Apr. 08, 2013BRL

Apr. 30, 2016Refinery

Sep. 30, 2015USD ($)

Jun. 30, 2015USD ($)

Mar. 31, 2015USD ($)

Jan. 31, 2014

Oct. 31, 2013USD ($)

Mar. 31, 2013USD ($)

Mar. 31, 2012USD ($)

Mar. 31, 2012BRL

Jun. 30, 2017USD ($)

Dec. 31, 2017EUR (€)

Dec. 31, 2017USD ($)

Dec. 31, 2016USD ($)

Dec. 31, 2015USD ($)

Dec. 31, 2014USD ($)

Dec. 31, 2012USD ($)

Dec. 31, 2009USD ($)

Dec. 31, 2002t

Dec. 31, 2017USD ($)

Dec. 31, 2017BRL

Dec. 31, 2017AUD

Aug. 31, 2017USD ($)

Dec. 31, 2016AUD

Apr. 08, 2013BRL

Mar. 31, 2013BRL

Jun. 30, 2012Installment

Loss Contingencies [Line Items]

Increase (decrease) in remediation reserve

$ 1,000,000

Payments related to remediation expenses applied against the reserve

$ 5,000,000

$ 24,000,000

Remediation expenses

8,000,000

26,000,000

$ 86,000,000

Revised project cost submitted to reflect the removal of a larger volume of contaminated soil

$ 7,000,000

Remediation reserve balance

324,000,000

$ 294,000,000

Purchase obligations due in 2018

176,000,000

Purchase obligations due in 2019

180,000,000

Purchase obligations due in 2020

186,000,000

Purchase obligations due in 2021

192,000,000

Purchase obligations due in 2022

199,000,000

Purchase obligations due thereafter

2,824,000,000

Purchase obligations expenditures

199,000,000

181,000,000

125,000,000

Number of alumina refineries to be powered under supplied agreement | Refinery

3

Number of installments | Installment

5

Asset included in other noncurrent assets

471,000,000

510,000,000

Operating leases, expense

101,000,000

90,000,000

98,000,000

Long-term operating leases, minimum annual rentals due in 2018

94,000,000

Long-term operating leases, minimum annual rentals due in 2019

73,000,000

Long-term operating leases, minimum annual rentals due in 2020

58,000,000

Long-term operating leases, minimum annual rentals due in 2021

47,000,000

Long-term operating leases, minimum annual rentals due in 2022

15,000,000

Long-term operating leases, minimum annual rentals due thereafter

21,000,000

Guarantees of third party related to project financing

177,000,000

Letters of credit, expiration date

2,018

Letters of credit, total amount committed

543,000,000

Outstanding bank guarantees and letters of credit

$ 29,000,000

Total amount committed under outstanding surety bonds

122,000,000

Surety bonds, expiration date

2,018

Tax Year 2006 Through 2009 [Member]

Loss Contingencies [Line Items]

Total combined assessments

€ 130

€ 130

155,000,000

AofA [Member] | Service Agreements [Member] | Alcoa Corporation [Member]

Loss Contingencies [Line Items]

Gas supply agreement period

12 years

12 years

Number of alumina refineries to be powered under supplied agreement | Refinery

3

Number of installments | Installment

2

Gas supply agreement prepayment amount

$ 500,000,000

AofA [Member] | Service Agreements [Member] | First Installment [Member] | Alcoa Corporation [Member]

Loss Contingencies [Line Items]

Gas supply agreement prepayment amount

200,000,000

Asset included in other noncurrent assets

471,000,000

510,000,000

AUD 654

AUD 654

AofA [Member] | Service Agreements [Member] | Second Installment [Member] | Alcoa Corporation [Member]

Loss Contingencies [Line Items]

Gas supply agreement prepayment amount

$ 200,000,000

Energy Raw Materials And Other Goods And Services [Member]

Loss Contingencies [Line Items]

Purchase obligations due in 2018

2,296,000,000

Purchase obligations due in 2019

1,946,000,000

Purchase obligations due in 2020

1,498,000,000

Purchase obligations due in 2021

1,343,000,000

Purchase obligations due in 2022

1,302,000,000

Purchase obligations due thereafter

11,433,000,000

State and Local Jurisdiction [Member]

Loss Contingencies [Line Items]

Total combined assessments

46,000,000

BRL 145,000,000

Income tax settled

8

BRL 25

Alcoa Corporation [Member]

Loss Contingencies [Line Items]

Total amount committed under outstanding surety bonds

14,000,000

Standby Letter of Credit Agreement [Member]

Loss Contingencies [Line Items]

Credit facility

$ 150,000,000

Letter of Credit [Member]

Loss Contingencies [Line Items]

Letters of credit

112,000,000

Baie Comeau Smelter [Member]

Loss Contingencies [Line Items]

Increase (decrease) in remediation reserve

$ 25,000,000

Fusina [Member]

Loss Contingencies [Line Items]

Increase (decrease) in remediation reserve

$ 0

$ 12,000,000

Remediation expenses

€ 7

$ 7,000,000

Portovesme [Member]

Loss Contingencies [Line Items]

Increase (decrease) in remediation reserve

$ 3,000,000

$ 3,000,000

Mosjoen [Member]

Loss Contingencies [Line Items]

Increase (decrease) in remediation reserve

$ 11,000,000

$ (2,000,000)

$ 12,000,000

$ 20,000,000

Remediation reserve balance

$ 8,000,000

2,000,000

East St. Louis, IL Site [Member]

Loss Contingencies [Line Items]

Remediation reserve balance

$ 4,000,000

Long-term inspection, maintenance, and monitoring program period in years

30 years

30 years

Alcoa World Alumina Brasil [Member]

Loss Contingencies [Line Items]

Disallowed tax credits

$ 110,000,000

BRL 220,000,000

Percentage of penalty of the gross disallowed amount

50.00%

50.00%

50.00%

Value added tax receivable

$ 41,000,000

BRL 82,000,000

Alcoa Aluminio [Member]

Loss Contingencies [Line Items]

Metal sold per month | t

2,000

Ligestra [Member]

Loss Contingencies [Line Items]

Costs and payments

€ 12

$ 17,000,000

Costs and payments related to damages

6

9,000,000

Payment period

10 years

10 years

Italian Ministry of Environment and Protection of Land and Sea [Member]

Loss Contingencies [Line Items]

Soil remediation project, estimated cost

24

33,000,000

Payments for emergency action and natural resource damages

€ 18

$ 25,000,000

Arconic Inc [Member]

Loss Contingencies [Line Items]

Outstanding bank guarantees and letters of credit

$ 18,000,000

Minimum [Member]

Loss Contingencies [Line Items]

Expiration date of unconditional purchase obligations for energy

2,028

2,028

Guarantees, expiration date

2,018

2,018

Minimum [Member] | Alcoa World Alumina Brasil [Member]

Loss Contingencies [Line Items]

Estimated range of reasonably possible loss, minimum

$ 0

Minimum [Member] | Alcoa World Alumina Brasil [Member] | Fixed Assets [Member]

Loss Contingencies [Line Items]

Disallowed tax credits

0

Minimum [Member] | Ligestra [Member]

Loss Contingencies [Line Items]

Percentage of payments and remediation costs

50.00%

Maximum [Member]

Loss Contingencies [Line Items]

Expiration date of unconditional purchase obligations for energy

2,037

2,037

Guarantees, expiration date

2,024

2,024

Maximum [Member] | Alcoa World Alumina Brasil [Member]

Loss Contingencies [Line Items]

Estimated range of reasonably possible loss, minimum

31,000,000

BRL 103,000,000

Maximum [Member] | Alcoa World Alumina Brasil [Member] | Fixed Assets [Member]

Loss Contingencies [Line Items]

Disallowed tax credits

$ 35,000,000

BRL 117,000,000

Maximum [Member] | Ligestra [Member]

Loss Contingencies [Line Items]

Percentage of payments and remediation costs

80.00%

Other Financial Information - S

Other Financial Information - Schedule of Interest Cost Components (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Financial Information [Abstract]
Amount charged to expense	$ 104	$ 243	$ 270
Amount capitalized	17	23	30
Interest costs, total	$ 121	$ 266	$ 300

Other Financial Information 129

Other Financial Information - Schedule of Other (Income) Expenses, Net (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Income and Expenses [Abstract]
Alcoa Corporation's equity in net income (loss) of affiliated companies	$ 28	$ 70	$ 89
Foreign currency losses (gains), net	8	8	(39)
Net gain from asset sales	(116)	(164)	(32)
Net loss on mark-to-market derivative instruments	24	9	26
Other, net	(2)	(12)	(2)
Other (income) expenses, net	$ (58)	$ (89)	$ 42

Other Financial Information - A

Other Financial Information - Additional Information (Detail) - USD ($) $ in Millions	Oct. 31, 2016	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Non operating Income Expense [Line Items]
Net gain from asset sales			$ 116	$ 164	$ 32
Increase (decrease) in restricted cash				(1,226)
Yadkin [Member]
Other Non operating Income Expense [Line Items]
Net gain from asset sales			$ 122
Alcoa Nederland Holding BV [Member]
Other Non operating Income Expense [Line Items]
Senior notes issued		$ 1,250
Net proceeds from issuance of senior notes		1,228
Restricted cash		$ 1,228
Increase (decrease) in restricted cash	$ (1,228)
Disposal of Equity Interests [Member]
Other Non operating Income Expense [Line Items]
Net gain from asset sales				27
Disposal of Land [Member]
Other Non operating Income Expense [Line Items]
Net gain from asset sales				$ 118	$ 29

Other Financial Information 131

Other Financial Information - Schedule of Other Noncurrent Assets (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Other Financial Information [Abstract]
Gas supply prepayment	$ 510	$ 471
Value-added tax credits	340	287
Prepaid gas transmission contract	300	270
Goodwill	154	155
Deferred mining costs, net	139	127
Prepaid pension benefit	72	43
Intangibles, net	62	135
Other	142	180
Other assets, noncurrent, total	$ 1,719	$ 1,668

Other Financial Information 132

Other Financial Information - Schedule of Other Noncurrent Assets (Parenthetical) (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2016USD ($)
AofA [Member]
Other Noncurrent Assets [Line Items]
Impaired interest amount	$ 72

Other Financial Information 133

Other Financial Information - Schedule of Other Noncurrent Liabilities and Deferred Credits (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Other Financial Information [Abstract]
Accrued compensation and retirement costs	$ 127	$ 122
Deferred alumina sales revenue	68	76
Liability related to the resolution of a legal matter		74
Other	84	98
Other noncurrent liabilities and deferred credits, total	$ 279	$ 370

Other Financial Information 134

Other Financial Information - Schedule of Other Noncurrent Liabilities and Deferred Credits (Parenthetical) (Detail) - Foreign Corrupt Practices Act Investigation [Member] $ in Millions	12 Months Ended
	Dec. 31, 2017USD ($)
Other Liabilities And Deferred Credits [Line Items]
Fines and penalties	$ 384
Fines and penalties paid	236
Fines and penalties payable	148
Fines and penalties installment amount	$ 74

Other Financial Information 135

Other Financial Information - Schedule of Cash Paid for Interest and Income Taxes (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Financial Information [Abstract]
Interest, net of amount capitalized	$ 100	$ 226	$ 270
Income taxes, net of amount refunded	$ 363	$ 265	$ 265

Subsequent Events - Additional

Subsequent Events - Additional Information (Detail) - Subsequent Event [Member] $ in Millions	Jan. 17, 2018USD ($)
Subsequent Event [Line Items]
Number of employees affected the change in defined contribution plans	800,000,000
Percentage of employers contribution in defined benefit plans	3.00%
Curtailment gain on net pension and other postretirement benefits	$ 20
Net pension and other postretirement benefits liability	35
Discretionary contributions of defined benefit pension plans	$ 300
Number of retirees covered under annuity contract	9,000,000,000

Quarterly Data - Schedule of Qu

Quarterly Data - Schedule of Quarterly Data (Detail) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Sales	$ 3,174	$ 2,964	$ 2,859	$ 2,655	$ 2,537	$ 2,329	$ 2,323	$ 2,129	$ 11,652	$ 9,318	$ 11,199
Net income (loss)	(56)	169	138	308	(129)	10	(12)	(215)	559	(346)	(739)
Net income (loss) attributable to Alcoa Corporation	(196)	113	75	225	(125)	(10)	(55)	(210)	217	(400)	(863)
Earnings per share attributable to Alcoa Corporation common shareholders(1):
Sales	3,174	2,964	2,859	2,655	2,537	2,329	2,323	2,129	11,652	9,318	11,199
Net (loss) income	(56)	169	138	308	(129)	10	(12)	(215)	559	(346)	(739)
Net loss attributable to Alcoa Corporation	$ (196)	$ 113	$ 75	$ 225	$ (125)	$ (10)	$ (55)	$ (210)	$ 217	$ (400)	$ (863)
Basic	$ (1.06)	$ 0.61	$ 0.41	$ 1.23	$ (0.68)	$ (0.06)	$ (0.29)	$ (1.16)	$ 1.18	$ (2.19)	$ (4.73)
Diluted	$ (1.06)	$ 0.60	$ 0.40	$ 1.21	$ (0.68)	$ (0.06)	$ (0.29)	$ (1.16)	$ 1.16	$ (2.19)	$ (4.73)

Quarterly Data - Schedule of138

Quarterly Data - Schedule of Quarterly Data (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Restructuring and other charges	$ 297	$ 209		$ 309	$ 318	$ 983
Discrete income tax charges for a valuation allowance	$ 98
Corporate tax rate	21.00%		21.00%	35.00%	35.00%	35.00%