Subsequent Events (Details) (USD $) In Millions	9 Months Ended
Sep. 30, 2011	Oct. 31, 2011
Subsequent Events [Line Items]
Subsequent Event, Date	Oct. 31, 2011
Subsequent Event, Description	As a result of a bankruptcy filing on October 31, 2011 by affiliated MF Global entities, the Chicago Mercantile Exchange froze all MF Global-related accounts (including MGE’s account).
Subsequent Event Customer Segregated Margin Amount		$ 3.3

Amount held at MF Global in a customer segregated margin account.

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Data Type:	xbrli:monetaryItemType
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- Definition

Describes the event or transaction that occurred between the balance sheet date and the date the financial statements are issued or available to be issued.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 5

-Paragraph 11

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 855

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6842918&loc=SL6314017-165662

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 165

-Paragraph 13

-Subparagraph a

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

+ Details

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- Definition

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.

+ Details

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- Definition

Date of subsequent event occurrence or of indication that subsequent event occurred.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 855

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6842918&loc=SL6314017-165662

+ Details

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Period Type:	duration