Pension and other postretirement benefits - Weighted Average assumptions used in calculating benefit obligations (Details)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Pension Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Discount rate	3.69%	4.19%
Rate of compensation increase, group 1	3.00%	3.00%
Rate of compensation increase, group 2	4.00%	4.00%
Measurement date	Dec. 31, 2017	Dec. 31, 2016
Other Postretirement Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans [Line Items]
Discount rate	3.66%	4.20%
Rate of compensation increase, group 1	3.00%	3.00%
Rate of compensation increase, group 2	4.00%	4.00%
Measurement date	Dec. 31, 2017	Dec. 31, 2016

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

No definition available.

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Namespace Prefix:	cenx_
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- Definition

Measurement Date, Benefit Obligation

+ References

No definition available.

+ Details

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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- Definition

Weighted average rate increase of compensation, used to determine benefit obligation 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/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(2)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

+ Details

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- 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

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- Details

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Namespace Prefix:
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- Details

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