XML 72 R55.htm IDEA: XBRL DOCUMENT v3.20.4

Fair Value Measurements - Significant Unobservable Inputs (Details) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) T	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
LME Swap [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 2.9	$ 10.6
LME Swap [Member] | Discounted Cash Flow [Member] | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, percentage	0.1000
Contingent obligation [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 0.0
Contingent obligation [Member] | Minimum [Member] | Discounted Cash Flow [Member] | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset (in tones) | T	1,975
Contingent obligation [Member] | Maximum [Member] | Discounted Cash Flow [Member] | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset (in tones) | T	2,348
FX Swap [Member]
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Fair value	$ 0.1	$ 0.0	$ (0.3)
FX Swap [Member] | Discounted Cash Flow [Member] | Level 3
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Derivative asset, percentage	10.00

X

- Definition

Value of input used to measure derivative asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

+ Details

Name:	us-gaap_DerivativeAssetMeasurementInput
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Nominal mass used to calculate payments on a derivative instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DerivativeNonmonetaryNotionalAmountMass
Namespace Prefix:	us-gaap_
Data Type:	num:massItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Fair value of financial instrument classified as derivative asset (liability) after deduction of derivative liability (asset), measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 3
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19279-110258

+ Details

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

X

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=cenx_FixedtoVariableLondonMetalsExchangeSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_ValuationTechniqueAxis=us-gaap_ValuationTechniqueDiscountedCashFlowMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=cenx_ContingentObligationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

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

X

- Details

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

X

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=cenx_FXSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: