Derivative Activities - Derivative Volumes Hedged and Average Hedge Prices (Parenthetical) (Detail) - Natural Gas - Purchased Put	Sep. 30, 2016 $ / MMBTU
2016
Derivative [Line Items]
Weighted average deferred premium	0.34
2017
Derivative [Line Items]
Weighted average deferred premium	0.32

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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