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Summary of Significant Accounting Policies - Additional Information (Detail) | 3 Months Ended | 12 Months Ended
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Mar. 31, 2018 USD ($) | Dec. 31, 2017 USD ($) | Dec. 31, 2015 USD ($) | Dec. 31, 2017 USD ($) reporting_unit | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Original maturity of cash equivalents |  |  |  | 3 months |  |
Number of reporting units | reporting_unit |  |  |  | 8 |  |
Segment allocation, goodwill recognized |  | $ 4,535,000,000 | $ 5,249,000,000 | $ 4,535,000,000 | $ 5,148,000,000 | $ 5,249,000,000
Minimum percentage of estimated fair value of reporting unit to be less than carrying amount of goodwill |  |  |  | 50.00% |  |
Period required for impairment testing of reporting units |  |  |  | 3 years |  |
Goodwill impairment |  |  |  | $ 719,000,000 | 0 | 25,000,000
Businesses expected to be disposed within, years |  |  |  | 1 year |  |
Adjustments for New Accounting Pronouncement | Pro Forma |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
New accounting pronouncement or change in accounting principle, effect of change on operating results |  |  |  | $ 154,000,000 | $ 135,000,000 |
Adjustments for New Accounting Pronouncement | Scenario, Forecast |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
New accounting pronouncement or change in accounting principle, effect of change on operating results | $ 20,000,000 |  |  |  |  |
Engineered Products and Solutions |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Number of reporting units | reporting_unit |  |  |  | 4 |  |
Number of reporting units for goodwill allocation | reporting_unit |  |  |  | 2 |  |
Number of reporting units being subjected to goodwill impairment test | reporting_unit |  |  |  | 6 |  |
Goodwill impairment |  | 719,000,000 |  | $ 719,000,000 |  |
Engineered Products and Solutions | Fastening systems and rings |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Segment allocation, goodwill recognized |  | 2,221,000,000 |  | 2,221,000,000 |  |
Engineered Products and Solutions | Alcoa Power and Propulsion |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Segment allocation, goodwill recognized |  | 1,686,000,000 |  | $ 1,686,000,000 |  |
Engineered Products and Solutions | Alcoa Forging and Extrusions |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Goodwill impairment |  | $ 719,000,000 |  |  |  |
Transportation and Construction Solutions |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Number of reporting units | reporting_unit |  |  |  | 3 |  |
Goodwill impairment |  |  |  |  |  | 25,000,000
Other Operating Segments |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Qualitative assessment for reporting units | reporting_unit |  |  |  | 1 |  |
Primary Metals |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Goodwill impairment |  |  | $ 25,000,000 |  |  |
Implied fair value of goodwill |  |  |  |  |  | $ 0
Minimum | Engineered Products and Solutions |  |  |  |  |  |
Summary Of Significant Accounting Policies [Line Items] |  |  |  |  |  |
Percentage of total goodwill allocated to two reporting units |  |  |  | 85.00% |  |

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