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Business acquisitions - Acquisition of Mt. Holly aluminum smelter (Details) - Mt. Holly Smelter [Member] - USD ($) $ / shares in Units, $ in Thousands | 10 Months Ended | 12 Months Ended |
--- | --- | --- | ---
Sep. 30, 2015 | Dec. 31, 2014 | Dec. 31, 2013 | Dec. 01, 2014
Assets Acquired: |  |  |  |
Inventories |  |  |  | $ 23,979
Due from Alumax |  |  |  | 11,269
Prepaid and other current assets |  |  |  | 2,527
Intangible asset |  |  |  | 2,580
Pension asset |  |  |  | 30,842
Property, plant and equipment – net |  |  |  | 142,837
Total assets acquired |  |  |  | 214,034
Liabilities Assumed: |  |  |  |
Accounts payable, trade |  |  |  | 41,471
Accrued and other current liabilities |  |  |  | 8,590
Accrued pension benefit costs |  |  |  | 34,595
Accrued postretirement benefit costs |  |  |  | 2,857
Asset retirement obligations |  |  |  | 8,213
Deferred taxes |  |  |  | 2,686
Total liabilities assumed |  |  |  | 98,412
Goodwill |  |  |  | 0
Business Combination, Provisional Information, Initial Accounting Incomplete, Adjustments [Abstract] |  |  |  |
Inventories | $ (2,126) |  |  |
Due from Alumax | (9,517) |  |  |
Property, plant and equipment – net | 15,748 |  |  |
Total assets acquired | 4,105 |  |  |
Accrued and other current liabilities | 255 |  |  |
Accrued pension benefit costs | 34,595 |  |  |
Deferred taxes | (2,118) |  |  |
Total liabilities assumed | 32,732 |  |  |
Goodwill | $ (4,804) |  |  |
Business Acquisition, Pro Forma Information [Abstract] |  |  |  |
Mt. Holly revenue |  | $ 25,911 |  |
Mt. Holly income from continuing operations |  | 3,024 |  |
Pro forma revenues |  | 2,176,552 | $ 1,707,838 |
Pro forma earnings from continuing operations |  | $ 125,847 | $ (38,819) |
Pro forma earnings per common share, basic (in dollars per share) |  | $ 1.30 | $ (0.44) |
Pro forma earnings per common share, diluted (in dollars per share) |  | $ 1.29 | $ (0.44) |
Preliminary estimate [Member] |  |  |  |
Assets Acquired: |  |  |  |
Inventories |  |  |  | 26,105
Due from Alumax |  |  |  | 20,786
Prepaid and other current assets |  |  |  | 2,527
Intangible asset |  |  |  | 2,580
Pension asset |  |  |  | 30,842
Property, plant and equipment – net |  |  |  | 127,089
Total assets acquired |  |  |  | 209,929
Liabilities Assumed: |  |  |  |
Accounts payable, trade |  |  |  | 41,471
Accrued and other current liabilities |  |  |  | 8,335
Accrued pension benefit costs |  |  |  | 0
Accrued postretirement benefit costs |  |  |  | 2,857
Asset retirement obligations |  |  |  | 8,213
Deferred taxes |  |  |  | 4,804
Total liabilities assumed |  |  |  | 65,680
Goodwill |  |  |  | $ 4,804

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