Subsequent events (Details) (USD $) In Thousands, unless otherwise specified			0 Months Ended
	Sep. 30, 2014 U.S. letter of credit [Member]	Oct. 23, 2014 Mt. Holly [Member] t	Oct. 23, 2014 U.S. letter of credit [Member]
Subsequent Event [Line Items]
Mt. Holly ownership percent acquired		50.30%
Mt. Holly ownership percent after acquisition		100.00%
Primary aluminum capacity - (in tonnes)		229,000
Purchase price for acquisition		$ 67,500
Debt Covenants, revised period (in days)			120 days
Letter of credit sub-facility amount	$ 80,000		$ 100,000