Derivatives (Natural Gas Gain Loss Included in Other Income and Expense) (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Derivative [Line Items]
Gain (loss) on contracts where hedge accounting was not elected	$ 31	$ (361)	$ (387)	$ (287)
Other Income (Expense) [Member]
Derivative [Line Items]
Gain (loss) on derivative, net	769	566	1,139	167
Natural Gas Contracts | Other Income (Expense) [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Gain (loss) on contracts where hedge accounting was not elected	769	58	1,139	(237)
Natural Gas Contracts | De-designated contracts [Member] | Other Income (Expense) [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Gain (loss) on derivative, net	$ 0	$ 508	$ 0	$ 404