Derivatives Natural Gas Gain Loss Included in Other Income and Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Derivative [Line Items]
Contracts where hedge accounting was not elected	$ (245)	$ (158)	$ 403
Other Income (Expense)
Derivative [Line Items]
Gain (loss) on derivative, net	1,860	218	(1,317)
Other Income (Expense) | Natural Gas Contracts | Cash Flow Hedging
Derivative [Line Items]
Contracts where hedge accounting was not elected	1,860	(714)	(81)
Other Income (Expense) | Natural Gas Contracts | De-designated contracts | Cash Flow Hedging
Derivative [Line Items]
Gain on Cash Flow Hedge Ineffectiveness	$ 0	$ 932
Loss on Cash Flow Hedge Ineffectiveness			$ (1,236)