Derivatives (Natural Gas Gain Loss Included in Other Income and Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Derivative [Line Items]
Gain (loss) on contracts where hedge accounting was not elected	$ 0	$ 106	$ 0	$ (281)
Natural Gas Contracts | Other Income (Expense) | Cash Flow Hedging
Derivative [Line Items]
Gain (loss) on contracts where hedge accounting was not elected	(4)	11	(823)	1,150
Gain (loss) on derivative, net	$ (4)	$ 11	$ (823)	$ 1,150