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Fair value measurements (Details) (USD $) In Thousands, unless otherwise specified | 3 Months Ended | 6 Months Ended |  |  |  |  |  |  |  |
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Jun. 30, 2014 Derivative Liabilities - Net [Member] | Jun. 30, 2013 Derivative Liabilities - Net [Member] | Jun. 30, 2014 Derivative Liabilities - Net [Member] | Jun. 30, 2013 Derivative Liabilities - Net [Member] | Jun. 30, 2014 Fair Value, Measurements, Recurring [Member] | Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] | Jun. 30, 2014 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member] | Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 1 [Member] | Jun. 30, 2014 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member] | Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 2 [Member] | Jun. 30, 2014 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member] | Dec. 31, 2013 Fair Value, Measurements, Recurring [Member] Fair Value, Inputs, Level 3 [Member]
ASSETS: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash equivalents |  |  |  |  | $ 51,172 |  | $ 49,658 |  | $ 51,172 |  | $ 49,658 |  | $ 0 |  | $ 0 |  | $ 0 |  | $ 0 |
Trust assets |  |  |  |  | 9,523 |  | 11,151 |  | 9,523 |  | 11,151 |  | 0 |  | 0 |  | 0 |  | 0 |
Surety bonds |  |  |  |  |  |  | 2,002 |  |  |  | 2,002 |  |  |  | 0 |  |  |  | 0 |
Midwest premium contracts |  |  |  |  |  |  | 140 |  |  |  | 0 |  |  |  | 0 |  |  |  | 140 |
TOTAL |  |  |  |  | 60,695 |  | 62,951 |  | 60,695 |  | 62,811 |  | 0 |  | 0 |  | 0 |  | 140 |
LIABILITIES: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
E.ON contingent obligation – net (1) |  |  |  |  | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1] | 0 | [1]
Primary aluminum sales contract |  |  |  |  |  |  | 140 |  |  |  | 0 |  |  |  | 0 |  |  |  | 140 |
TOTAL |  |  |  |  | 0 |  | 140 |  | 0 |  | 0 |  | 0 |  | 0 |  | 0 |  | 140 |
Level 3 fair value measurements [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Beginning balance, period start | 0 | (1,030) | 0 | (16,539) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Total gain (loss) included in earnings | 0 | 257 | (940) | 15,766 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Settlements |  |  | 940 | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Ending balance, June 30, | 0 | (773) | 0 | (773) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of gain (loss) included in earnings attributable to the change in unrealized losses relating to assets and liabilities held at June 30, | $ 0 | $ 257 | $ 140 | $ 15,766 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | See Note 9 Debt for additional information about the E.ON contingent obligation.

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