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Business acquisitions - Acquisition of Mt. Holly smelter narrative (Details) - USD ($) |  | 1 Months Ended | 3 Months Ended | 6 Months Ended
--- | --- | --- | --- | ---
Dec. 01, 2014 | Mar. 31, 2016 | Apr. 30, 2015 | Jun. 30, 2016 | Jun. 30, 2015 | Jun. 30, 2016 | Jun. 30, 2015
Business Acquisition [Line Items] |  |  |  |  |  |  |
Gains recognized on contingent consideration |  |  |  | $ 0 | $ 10,287,000 | $ 0 | $ 16,814,000
Total net consideration paid | $ 41,487 |  |  |  |  |  |
Mt. Holly Smelter [Member] |  |  |  |  |  |  |
Business Acquisition [Line Items] |  |  |  |  |  |  |
Purchase price | 67,500,000 |  |  |  |  |  |
Non-cash pre-tax gain at closing | 15,955,000 |  |  |  |  |  |
Acquisition costs | 1,539,000 |  |  |  |  |  |
Consideration received for expected future post-employment benefits | $ 2,400,000 |  |  |  |  |  |
Consideration received for working capital adjustments |  |  | $ 11,313,000 |  |  |  |
Proceeds from acquisition - contingent consideration |  | $ 12,500,000 |  |  |  |  |
Gains recognized on contingent consideration |  |  |  |  | $ 10,287,000 |  | $ 16,814,000

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- Definition The revenues and gains recognized recorded for each transaction with the acquiree that is recognized separately from the acquisition of assets and assumptions of liabilities in the business combination. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (e)(3) -URI http://asc.fasb.org/extlink&oid=65885119&loc=d3e1392-128463 + Details
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