Derivative Instruments (Fair Values of Derivative Instruments in Consolidated Balance Sheets) (Details) - Not Designated as Hedging Instrument [Member] - Commodity Forward Contracts [Member]	3 Months Ended	12 Months Ended
Mar. 31, 2017 MMBTU gal	Dec. 31, 2016 MMBTU gal
Derivatives, Fair Value [Line Items]
Derivative, Nonmonetary Notional Amount, Volume \| gal	221,800,000	257,000,000
Derivative, Nonmonetary Notional Amount, Energy Measure \| MMBTU	8,800,000	8,500,000

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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