Derivatives (Natural Gas Gain Loss Included in Other Income and Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Derivative [Line Items]
Gain (loss) on contracts where hedge accounting was not elected	$ 106	$ 135	$ (281)	$ (152)
Other Income (Expense) [Member]
Derivative [Line Items]
Gain (loss) on derivative, net	11	(42)	1,150	125
Natural Gas Contracts | Other Income (Expense) [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Gain (loss) on contracts where hedge accounting was not elected	11	(222)	1,150	(459)
Natural Gas Contracts | De-designated contracts [Member] | Other Income (Expense) [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Gain (loss) on derivative, net	$ 0	$ 180	$ 0	$ 584