Derivatives Natural Gas Gain Loss Included in Other Income and Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Natural Gas Contracts | Designated contracts with ineffectiveness | Cash Flow Hedging
Derivative [Line Items]
Loss on Cash Flow Hedge Ineffectiveness			$ (300)
Other Income (Expense)
Derivative [Line Items]
Gain (loss) on derivative, net	$ 218	$ (1,317)	(276)
Other Income (Expense) | Natural Gas Contracts | Designated contracts with ineffectiveness | Cash Flow Hedging
Derivative [Line Items]
Gain (loss) on derivative, net	0	0	(276)
Other Income (Expense) | Natural Gas Contracts | De-designated contracts
Derivative [Line Items]
Contracts where hedge accounting was not elected	(714)	(81)	0
Other Income (Expense) | Natural Gas Contracts | De-designated contracts | Cash Flow Hedging
Derivative [Line Items]
Gain on Cash Flow Hedge Ineffectiveness	$ 932
Loss on Cash Flow Hedge Ineffectiveness		$ (1,236)	$ 0