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Subsequent Events (Details) - Subsequent Event - USD ($) $ / shares in Units, shares in Millions | Feb. 21, 2019 | Feb. 08, 2019 | Feb. 19, 2019
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Subsequent Event [Line Items] |  |  |
Stock repurchase program, authorized amount |  | $ 500,000,000 |
Stock repurchase program, additional amount authorized |  | $ 500,000,000 |
Minimum |  |  |
Subsequent Event [Line Items] |  |  |
Dividends payable, planned decrease (USD per share) |  | $ 0.06 |
Maximum |  |  |
Subsequent Event [Line Items] |  |  |
Dividends payable, planned decrease (USD per share) |  | $ 0.02 |
Accelerated Share Repurchase Program |  |  |
Subsequent Event [Line Items] |  |  |
Stock repurchase program, authorized amount |  |  | $ 700,000,000
Initial number of shares received | 32 |  |

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- Definition Accelerated Share Repurchase, Initial Shares Received + References No definition available. + Details
Name: | arnc_AcceleratedShareRepurchaseInitialSharesReceived
Namespace Prefix: | arnc_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

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- Definition Dividends Payable, Planned Decrease + References No definition available. + Details
Name: | arnc_DividendsPayablePlannedDecrease
Namespace Prefix: | arnc_
Data Type: | num:perShareItemType
Balance Type: | na
Period Type: | duration

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- Definition Stock Repurchase Program, Additional Shares Authorized + References No definition available. + Details
Name: | arnc_StockRepurchaseProgramAdditionalSharesAuthorized
Namespace Prefix: | arnc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Amount of stock repurchase plan authorized. + References No definition available. + Details
Name: | us-gaap_StockRepurchaseProgramAuthorizedAmount1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event. + References No definition available. + Details
Name: | us-gaap_SubsequentEventLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_ShareRepurchaseProgramAxis=arnc_AcceleratedShareRepurchaseProgramMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |