Derivative Instruments (BalanceSheet) (Details) (Designated as Hedging Instrument, USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Assets
Gross derivative position, assets	$ 6,988	$ 620
Liabilities
Gross derivative position, liability		(422)
Energy derivative contract | Assets from price risk management - current
Assets
Gross derivative position, assets	3,546	218
Energy derivative contract | Assets from price risk management - long term
Assets
Gross derivative position, assets	3,442	402
Energy derivative contract | Liabilities from price risk management - current
Liabilities
Gross derivative position, liability		(13)
Energy derivative contract | Liabilities from price risk management long term
Liabilities
Gross derivative position, liability		(97)
Interest Rate Swap | Other current liabilities
Liabilities
Gross derivative position, liability		(222)
Interest Rate Swap | Other long-term liabilities
Liabilities
Gross derivative position, liability		$ (90)