Derivatives (Narrative - Commodity Future Contracts) (Details) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015 USD ($) MMBTU	Sep. 30, 2014 USD ($)	Sep. 30, 2015 USD ($) MMBTU	Sep. 30, 2014 USD ($)	Dec. 31, 2014 MMBTU
Derivative [Line Items]
Contracts where hedge accounting was not elected	$ 135	$ 461	$ (152)	$ 274
Cash Flow Hedging [Member] | Natural Gas Contracts
Derivative [Line Items]
Forecast of commodity requirements, maximum length of time used			18 months
Natural gas contracts, notional amounts (in millions of BTUs) | MMBTU	3,120,000		3,120,000		3,850,000
Derivative, Additional Cash Paid on Settlement of Hedge	$ (1,000)	(100)	$ (3,200)
Derivative, Cash Received on Hedge				800
Cash flow hedge loss to be reclassified within 12 months			$ 1,100
Minimum [Member] | Cash Flow Hedging [Member] | Natural Gas Contracts
Derivative [Line Items]
Forecast of anticipated requirements, percentage of forecast eligible for hedging	40.00%		40.00%
Maximum [Member] | Cash Flow Hedging [Member] | Natural Gas Contracts
Derivative [Line Items]
Forecast of anticipated requirements, percentage of forecast eligible for hedging	70.00%		70.00%
Other Income (Expense) [Member]
Derivative [Line Items]
Gain (loss) on derivative, net	$ (42)	0	$ 125	0
Other Income (Expense) [Member] | Cash Flow Hedging [Member] | Natural Gas Contracts
Derivative [Line Items]
Contracts where hedge accounting was not elected	(222)	0	(459)	0
Other Income (Expense) [Member] | Not Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Natural Gas Contracts
Derivative [Line Items]
Gain (loss) on derivative, net	$ 180	$ 0	$ 584	$ 0