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Commitments and contingencies - Legal Contingencies (Details) - USD ($) |  | 1 Months Ended | 3 Months Ended | 12 Months Ended | 45 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Feb. 09, 2017 | Mar. 31, 2015 | Apr. 30, 2013 | Mar. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2016 | Sep. 30, 2016
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Pension contributions |  |  |  |  | $ 0 | $ 34,595,000 | $ 0 |  |  |
PBGC Agreement [Member] |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Required pension contributions above minimum |  |  | $ 17,400,000 |  |  |  |  |  | $ 17,400,000 |
Pension contributions |  | $ 1,100,000 |  |  | 0 | $ 1,100,000 | $ 0 | $ 6,700,000 |  |
Pension contributions above minimum remaining |  |  |  | $ 9,600,000 | $ 9,600,000 |  |  |  |  |
Hawesville [Member] |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Power supply agreement, termination notice period |  |  |  | 1 year | 1 year |  |  |  |  |
Ravenswood Retiree Medical Benefits Changes [Member] |  |  |  |  |  |  |  |  |  |
Loss Contingencies [Line Items] |  |  |  |  |  |  |  |  |  |
Litigation settlement amount | $ 23,000,000 |  |  |  |  |  |  |  |  |
Estimated litigation liability |  |  |  |  | $ 23,000,000 |  |  |  | $ 23,000,000 | $ 23,000,000
Litigation settlement period | 10 years |  |  |  |  |  |  |  |  |
Litigation settlement payable upon final approval of settlement | $ 5,000,000 |  |  |  |  |  |  |  |  |
Litigation settlement, annual installment | $ 2,000,000 |  |  |  |  |  |  |  |  |
Litigation settlement installment period | 9 years |  |  |  |  |  |  |  |  |

X
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- Definition Litigation Settlement, Payable upon Final Approval of Settlement + References No definition available. + Details
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- Definition Aggregate carrying amount of the estimated litigation liability for known or estimated probable loss from litigation, which may include attorneys' fees and other litigation costs. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 450 -SubTopic 20 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=25496072&loc=d3e14326-108349 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.25) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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