Derivatives (Details) € in Millions	9 Months Ended
Sep. 30, 2020 EUR (€) MWh t
LME Swap [Member]
Derivative [Line Items]
Other forward delivery contracts to sell primary aluminum, open (in tonnes)	124,378
Midwest Premium (MWP) [Member]
Derivative [Line Items]
Open position to offset fixed prices	126,000
Fixed For Floating Swaps [Member]
Derivative [Line Items]
Open position to offset fixed prices	13,144
Hawesville L4 Power Price Swaps [Member]
Derivative [Line Items]
Power available \| MWh	198,810
Nordpool Power Price Swap [Member] \| Grundartangi [Member]
Derivative [Line Items]
Power available \| MWh	1,373,790
FX Swap [Member]
Derivative [Line Items]
Derivative asset \| €	€ 34.6

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 55
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=99393222&loc=SL20226008-175313

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 825
-SubTopic 10
-Section 50
-Paragraph 10
-URI http://asc.fasb.org/extlink&oid=121572278&loc=d3e13433-108611

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=51824906&loc=SL20225862-175312

Name:	us-gaap_DerivativeAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant