Derivative Instruments (Narrative) (Details) - Not Designated as Hedging Instrument [Member] - Commodity Forward Contracts [Member]	9 Months Ended	12 Months Ended
	Sep. 30, 2017 MMBTU gal	Dec. 31, 2016 MMBTU gal
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount, Volume | gal	138,600,000	257,000,000
Derivative, Nonmonetary Notional Amount, Energy Measure | MMBTU	12,500,000	8,500,000