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Fair value measurements (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended |  |  |  |  |  |  |  |  |
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Mar. 31, 2013 | Mar. 31, 2013 Derivative Liabilities - Net [Member] | Mar. 31, 2012 Derivative Liabilities - Net [Member] | Dec. 31, 2010 Derivative Liabilities - Net [Member] | Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] | Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] | Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member] | Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member] | Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member] | Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member] | Mar. 31, 2013 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member] | Dec. 31, 2012 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Derivative Instruments, Gain (Loss) Reclassification from Accumulated OCI to Income, Estimated Net Amount to be Transferred | $ 153 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
ASSETS: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash equivalents |  |  |  |  | 182,844 |  | 168,309 |  | 182,844 |  | 168,309 |  | 0 |  | 0 |  | 0 |  | 0 |
Trust assets |  |  |  |  | 13,433 | [1] | 14,254 | [1] | 13,433 | [1] | 14,254 | [1] | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1]
Surety bond – workers comp insurance |  |  |  |  | 2,123 |  | 2,123 |  | 2,123 |  | 2,123 |  | 0 |  | 0 |  | 0 |  | 0 |
TOTAL |  |  |  |  | 198,400 |  | 184,686 |  | 198,400 |  | 184,686 |  | 0 |  | 0 |  | 0 |  | 0 |
LIABILITIES: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
E.ON contingent obligation – net |  |  |  |  | 0 | [2] | 15,369 |  | 0 | [2] | 0 |  | 0 | [2] | 0 |  | 0 | [2] | 15,369 |
Primary aluminum sales contract – premium collar |  |  |  |  | 1,030 |  | 1,170 |  | 0 |  | 0 |  | 0 |  | 0 |  | 1,030 |  | 1,170 |
TOTAL |  |  |  |  | 1,030 |  | 16,539 |  | 0 |  | 0 |  | 0 |  | 0 |  | 1,030 |  | 16,539 |
Level 3 fair value measurements [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance |  | (16,539) |  | (14,760) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total gain (realized/unrealized) included in earnings |  | 15,509 | (613) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlements |  | 0 | (53) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending balance |  | (1,030) | (15,426) | (14,760) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of total loss included in earnings attributable to the change in unrealized losses (gains) relating to assets and liabilities held |  | $ (15,509) | $ 613 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | Trust assets are currently invested in money market funds and excludes the cash surrender value of the life insurance policies in the trust.
[2] | Based on the LME forward market prices for primary aluminum at March 31, 2013 and management's estimate of the LME forward market for periods beyond the quoted periods, we will not have any payment obligations for the E.ON contingent obligation through the term of the agreement which expires in 2028. See Note 9 Debt for additional information about the E.ON contingent obligation fair value.

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