Derivatives (Narrative - Commodity Future Contracts) (Details) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015 USD ($) MMBTU	Jun. 30, 2014 USD ($)	Jun. 30, 2015 USD ($) MMBTU	Jun. 30, 2014 USD ($)	Dec. 31, 2014 MMBTU
Derivative [Line Items]
Contracts where hedge accounting was not elected	$ (361)	$ (187)	$ (287)	$ (187)
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	2,850,000		2,850,000		3,850,000
Derivative, Additional Cash Paid on Settlement of Hedge	$ 1,000		$ 2,200
Derivative, Cash Received on Hedge		300		800
Cash flow hedge loss to be reclassified within 12 months			$ 1,200
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	$ 566	0	$ 167	0
Other Income (Expense) [Member] | Cash Flow Hedging [Member] | Natural Gas Contracts
Derivative [Line Items]
Contracts where hedge accounting was not elected	58	0	(237)	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	$ 508	$ 0	$ 404	$ 0