Derivatives (Narrative - Natural Gas Contracts) (Details) - Cash Flow Hedging - Natural Gas Contracts $ in Millions	12 Months Ended
	Dec. 31, 2016 USD ($) MMBTU	Dec. 31, 2015 USD ($) MMBTU	Dec. 31, 2014 USD ($)
Derivative [Line Items]
Forecast of commodity requirements, maximum length of time used	18 months
Natural gas contracts, notional amounts (in millions of BTUs) | MMBTU	2,590,000	3,000,000
Derivative,cash paid on settlement of hedge	$ (2.3)	$ (4.6)
Derivative, cash received on settlement of hedge			$ 0.6
Cash flow hedge gain to be reclassified within 12 months	$ 0.7
Minimum [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, percent of anticipated requirements	40.00%
Maximum [Member]
Derivative [Line Items]
Derivative, nonmonetary notional amount, percent of anticipated requirements	70.00%