Derivatives (Natural Gas Gain Loss Included in Other Income and Expense) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Derivative [Line Items]
Gain (loss) on contracts where hedge accounting was not elected	$ 0	$ (418)
Other Income (Expense) [Member]
Derivative [Line Items]
Gain (loss) on derivative, net	(583)	370
Natural Gas Contracts | Other Income (Expense) [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Gain (loss) on contracts where hedge accounting was not elected	$ (583)	$ 370