Derivatives Natural Gas Gain Loss Included in Other Income and Expense (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Derivative [Line Items]
Contracts where hedge accounting was not elected	$ 403	$ (41)	$ (24)
Other Income (Expense)
Derivative [Line Items]
Gain (loss) on derivative, net	(1,317)	(276)	0
Other Income (Expense) | Natural Gas Contracts | Cash Flow Hedging
Derivative [Line Items]
Contracts where hedge accounting was not elected	(81)	0	0
Other Income (Expense) | Natural Gas Contracts | Designated contracts with ineffectiveness | Cash Flow Hedging
Derivative [Line Items]
Gain (loss) on derivative, net	0	(276)	0
Other Income (Expense) | Natural Gas Contracts | De-designated contracts | Cash Flow Hedging
Derivative [Line Items]
Gain (loss) on derivative, net	$ (1,236)	$ 0	$ 0