Fair value measurements (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013 Derivative Liabilities - Net [Member]	Sep. 30, 2012 Derivative Liabilities - Net [Member]	Sep. 30, 2013 Derivative Liabilities - Net [Member]	Sep. 30, 2012 Derivative Liabilities - Net [Member]	Sep. 30, 2013 Fair Value, Measurements, Recurring [Member]		Dec. 31, 2012 Fair Value, Measurements, Recurring [Member]		Sep. 30, 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]		Sep. 30, 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]		Sep. 30, 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]
ASSETS:
Cash equivalents					$ 119,183		$ 168,309		$ 119,183		$ 168,309		$ 0		$ 0		$ 0		$ 0
Trust assets (1)					11,588	[1]	14,254	[1]	11,588	[1]	14,254	[1]	0	[1]	0	[1]	0	[1]	0	[1]
Surety bonds					1,574		2,123		1,574		2,123		0		0		0		0
Midwest premium contracts					363				0				0				363
TOTAL					132,708		184,686		132,345		184,686		0		0		363		0
LIABILITIES:
E.ON contingent obligation – net (2)					0	[2]	15,369		0	[2]	0		0	[2]	0		0	[2]	15,369
Primary aluminum sales contract					771		1,170		0		0		0		0		771		1,170
TOTAL					771		16,539		0		0		0		0		771		16,539
Level 3 fair value measurements [Roll Forward]
Beginning balance, January 1,	(773)	(16,024)	(16,539)	(14,760)
Total gain (loss) included in earnings	365	(371)	16,131	(1,529)
Settlements	0	0	0	(106)
Ending balance, September 30,	(408)	(16,395)	(408)	(16,395)
Amount of gain (loss) included in earnings attributable to the change in unrealized losses relating to assets and liabilities held at September 30,	$ 365	$ (371)	$ 16,131	$ (1,529)

[1] Trust assets are currently invested in money market funds.

[2] Based on the LME forward market prices for primary aluminum at September 30, 2013 and management's estimate of the LME forward market for periods beyond the quoted periods, we believe that 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 10 Debt for additional information about the E.ON contingent obligation fair value.