Derivatives (Open Commodity Derivative Contracts - Natural Gas) (Details)	Dec. 31, 2016 MMBTU
Over the Counter \| Long
Derivative [Line Items]
Notional contracts (in MMbtu)	297,017

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.

No definition available.

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