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Fair value measurements (Tables)

12 Months Ended

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