Derivatives Natural Gas Gain Loss Included in Other Income and Expense (Details) (Other Income (Expense) [Member], USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Derivative [Line Items]
Gain (loss) on derivative, net	$ (399)	$ 0
Natural Gas Contracts | Cash Flow Hedging [Member]
Derivative [Line Items]
Contracts where hedge accounting was not elected	(295)	0
Natural Gas Contracts | De-designated contracts [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Gain (loss) on derivative, net	$ (104)	$ 0