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Financial Instruments (Summary of the Sales Activity for Investment Securities) (Details) (USD $) In Millions, unless otherwise specified | 12 Months Ended
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Dec. 31, 2011 | Dec. 31, 2010 | Dec. 31, 2009
Proceeds from Sales of Available-for-Sale Securities | $ 29 | [1] | $ 1 | [1] | $ 53 | [1]
Available-for-sale Securities, Gross Realized Gains |  |  |  |  | 8 |
Other Comprehensive Income Reclassifcation Adjustment for Sale of Securities Included in Net Income Net of Tax |  |  |  |  | 5 |
Gross Realized Losses from Sales of Available-for-Sale Securities Less Than | 1 |  | 1 |  | 1 |
Consumers Energy Company [Member] |  |  |  |  |  |
Proceeds from Sales of Available-for-Sale Securities | 19 | [1] |  |  | 32 | [1]
Available-for-sale Securities, Gross Realized Gains |  |  |  |  | 5 |
Other Comprehensive Income Reclassifcation Adjustment for Sale of Securities Included in Net Income Net of Tax |  |  |  |  | 3 |
Gross Realized Losses from Sales of Available-for-Sale Securities Less Than | $ 1 |  | $ 1 |  | $ 1 |
[1] | All proceeds related to sales of investments that were held within the SERP and classified as available for sale. Realized losses on these sales were less than $1 million for both CMS Energy and Consumers during each period.

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- Definition The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 21 -Subparagraph a -LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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. Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=6872113&loc=d3e27357-111563 + Details
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