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Fair value measurements (Tables) | 12 Months Ended
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Dec. 31, 2017
Fair Value Disclosures [Abstract] |
Schedule of financial assets and liabilities at fair value on a recurring basis |
Recurring Fair Value Measurements |  | As of December 31, 2017
|  | Level 1 |  | Level 2 |  | Level 3 |  | Total
ASSETS: |  |  |  |  |  |  |  |
Cash equivalents |  | $ | 142,819 |  |  | $ | — |  |  | $ | — |  |  | $ | 142,819 |
Trust assets (1) |  | 1,757 |  |  | — |  |  | — |  |  | 1,757 |
Surety bonds |  | 1,618 |  |  | — |  |  | — |  |  | 1,618 |
Derivative instruments |  | — |  |  | — |  |  | 1,724 |  |  | 1,724 |
TOTAL |  | $ | 146,194 |  |  | $ | — |  |  | $ | 1,724 |  |  | $ | 147,918 |
LIABILITIES: |  |  |  |  |  |  |  |  |  |  |  |
Contingent obligation – net (2) |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |
Derivative instruments |  | — |  |  | — |  |  | 1,200 |  |  | 1,200 |
TOTAL |  | $ | — |  |  | $ | — |  |  | $ | 1,200 |  |  | $ | 1,200 |
Recurring Fair Value Measurements |  | As of December 31, 2016
|  | Level 1 |  | Level 2 |  | Level 3 |  | Total
ASSETS: |  |  |  |  |  |  |  |
Cash equivalents |  | $ | 79,014 |  |  | $ | — |  |  | $ | — |  |  | $ | 79,014 |
Trust assets (1) |  | 3,147 |  |  | — |  |  | — |  |  | 3,147 |
Surety bonds |  | 1,874 |  |  | — |  |  | — |  |  | 1,874 |
Derivative instruments |  | — |  |  | — |  |  | 925 |  |  | 925 |
TOTAL |  | $ | 84,035 |  |  | $ | — |  |  | $ | 925 |  |  | $ | 84,960 |
LIABILITIES: |  |  |  |  |  |  |  |  |  |  |  |
Contingent obligation – net (2) |  | $ | — |  |  | $ | — |  |  | $ | — |  |  | $ | — |
Derivative instruments |  | — |  |  | — |  |  | 253 |  |  | 253 |
TOTAL |  | $ | — |  |  | $ | — |  |  | $ | 253 |  |  | $ | 253 |
(1) | Trust assets are currently invested in money market funds. These trust assets are held to fund the non-qualified supplemental executive pension benefit obligations for certain of our officers.
(2) | See Note 5 Debt for additional information about the contingent obligation.
Schedule of valuation methodology for assets and liabilities at fair value | The following section describes the valuation techniques and inputs used for fair value measurements categorized within Level 2 or Level 3 of the fair value hierarchy:
Level 2 and Level 3 Fair Value Measurements:
Asset / Liability |  | Level |  | Valuation Techniques |  | Inputs
Fixed for floating swaps |  | 3 |  | Discounted cash flows |  | Quoted LME forward market, management's estimates of future U.S. Midwest premium
Power price swap |  | 3 |  | Discounted cash flows |  | Quoted Nordpool forward market, discount rate
Contingent obligation |  | 3 |  | Discounted cash flows |  | Quoted LME forward market, management’s estimates of the LME forward market prices for periods beyond the quoted periods, management’s estimate of future level of operations and discount rate

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- Definition Tabular disclosure of the inputs and valuation techniques used to measure fair value, and a discussion of changes in valuation techniques and related inputs, if any, applied during the period to each separate class of assets, liabilities, and financial instruments classified in shareholders' equity that are measured on a recurring and/or nonrecurring basis. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb) -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19207-110258 + Details
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- Definition Tabular disclosure of assets and liabilities, including [financial] instruments measured at fair value that are classified in stockholders' equity, if any, that are measured at fair value on a recurring basis. The disclosures contemplated herein include the fair value measurements at the reporting date by the level within the fair value hierarchy in which the fair value measurements in their entirety fall, segregating fair value measurements using quoted prices in active markets for identical assets (Level 1), significant other observable inputs (Level 2), and significant unobservable inputs (Level 3). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=66048111&loc=d3e19190-110258 + Details
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