Derivatives (Natural Gas Gain Loss Included in Other Income and Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Derivative [Line Items]
Gain (loss) on contracts where hedge accounting was not elected	$ 0	$ 31	$ 0	$ (387)
Natural Gas Contracts | Other Income (Expense) | Cash Flow Hedging
Derivative [Line Items]
Gain (loss) on contracts where hedge accounting was not elected	(236)	769	(819)	1,139
Gain (loss) on derivative, net	$ (236)	$ 769	$ (819)	$ 1,139