XML 92 R75.htm IDEA: XBRL DOCUMENT v3.20.4

Pension and Other Postretirement Benefits - Weighted average assumptions used to determine net periodic benefit cost (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Rate of compensation increase, year one	3.00%	3.00%
Rate of compensation increase, year one and thereafter	3.50%	3.50%
Rate of compensation increase, year two			3.00%
Rate of compensation increase, year two and thereafter			4.00%
Pension
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Measurement date	12/31/2019	12/31/2018	12/31/2017
Fiscal year end	12/31/2020	12/31/2019	12/31/2018
Discount rate	3.22%	4.38%	3.69%
Rate of compensation increase, group 1	3.00%	3.00%	3.00%
Rate of compensation increase, group 2	3.50%	3.50%	4.00%
Expected return on plan assets	7.25%	7.25%	7.18%
OPEB
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Measurement date	12/31/2019	12/31/2018	12/31/2017
Fiscal year end	12/31/2020	12/31/2019	12/31/2018
Discount rate	2.79%	4.29%	3.66%
Rate of compensation increase, group 1	3.00%	3.00%	3.00%
Rate of compensation increase, group 2	3.50%	3.50%	4.00%
Expected return on plan assets	0.00%	0.00%	0.00%

X

- Definition

The maximum expected rate of compensation increases (for pay-related plans).

+ References

No definition available.

+ Details

Name:	cenx_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncreaseMax
Namespace Prefix:	cenx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year One

+ References

No definition available.

+ Details

Name:	cenx_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateofCompensationIncreaseYearOne
Namespace Prefix:	cenx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year One And Thereafter

+ References

No definition available.

+ Details

Name:	cenx_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateofCompensationIncreaseYearOneAndThereafter
Namespace Prefix:	cenx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year Two

+ References

No definition available.

+ Details

Name:	cenx_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateofCompensationIncreaseYearTwo
Namespace Prefix:	cenx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Defined Benefit Plan, Assumptions Used Calculating Net Periodic Benefit Cost, Rate of Compensation Increase, Year Two And Thereafter

+ References

No definition available.

+ Details

Name:	cenx_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateofCompensationIncreaseYearTwoAndThereafter
Namespace Prefix:	cenx_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Year end for fiscal year

+ References

No definition available.

+ Details

Name:	cenx_FiscalYearEnd
Namespace Prefix:	cenx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

The date(s) used to measure (at least the majority of) plan assets and benefit obligations for defined benefit pension and other postretirement benefit plans.

+ References

No definition available.

+ Details

Name:	cenx_MeasurementDateNetPeriodicBenefitCost
Namespace Prefix:	cenx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(1)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostDiscountRate
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostExpectedLongTermReturnOnAssets
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(2)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

+ Details

Name:	us-gaap_DefinedBenefitPlanAssumptionsUsedCalculatingNetPeriodicBenefitCostRateOfCompensationIncrease
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
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_DefinedBenefitPlansAndOtherPostretirementBenefitPlansTableTextBlockLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_PensionPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RetirementPlanTypeAxis=us-gaap_OtherPostretirementBenefitPlansDefinedBenefitMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: