Derivatives (Narrative - Natural Gas Contracts) (Details) - Natural Gas Contracts $ in Thousands	12 Months Ended
	Dec. 31, 2015 USD ($) MMBTU	Dec. 31, 2014 USD ($) MMBTU	Dec. 31, 2013 USD ($)
Cash Flow Hedging
Derivative [Line Items]
Forecast of commodity requirements, maximum length of time used	18 months
Natural gas contracts, notional amounts (in millions of BTUs) | MMBTU	3,000,000	3,850,000
Derivative, cash received on settlement of hedge		$ 600
Derivative,cash paid on settlement of hedge	$ (4,600)		$ (100)
Cash flow hedge loss to be reclassified within 12 months	$ 1,100
Minimum [Member] | Cash Flow Hedging
Derivative [Line Items]
Derivative, nonmonetary notional amount, percent of anticipated requirements	40.00%
Maximum [Member] | Cash Flow Hedging
Derivative [Line Items]
Derivative, nonmonetary notional amount, percent of anticipated requirements	70.00%
Designated as Hedging Instrument | Cash Flow Hedging
Derivative [Line Items]
Loss on Cash Flow Hedge Ineffectiveness			300
Other Income (Expense) | Not Designated as Hedging Instrument
Derivative [Line Items]
Derivative, Loss on Derivative	$ 714	81	0
Other Income (Expense) | Not Designated as Hedging Instrument | Cash Flow Hedging
Derivative [Line Items]
Gain on Cash Flow Hedge Ineffectiveness	$ 932
Loss on Cash Flow Hedge Ineffectiveness		$ 1,236	$ 0