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=109250915&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

Derivatives (Details) € in Millions, $ in Millions	12 Months Ended
Derivatives (Details) € in Millions, $ in Millions	Dec. 31, 2018 USD ($) MWh t	Dec. 31, 2017 USD ($)	Dec. 31, 2018 EUR (€) MWh t
Supply Commitment [Line Items]
Derivative asset \| $	$ 8.2	$ 1.7
Fixed to Variable London Metals Exchange Swap [Member]
Supply Commitment [Line Items]
Other forward delivery contracts to sell primary aluminum (in tonnes)	58,106		58,106
Midwest Premium (MWP) [Member]
Supply Commitment [Line Items]
Open position to offset fixed prices	36,000
Grundartangi [Member] \| Power Price Swap [Member]
Supply Commitment [Line Items]
Derivative, financial contract, percentage		4.00%
Underlying, derivative energy, available \| MWh	256,200		256,200
Derivative asset \| €			€ 2.8