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Financial Instruments (Schedule of Investment Securities) (Details) (USD $) In Millions |  |  | 9 Months Ended |  |  |  |  |  |  |  |  |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Sep. 30, 2011 | Dec. 31, 2010 | Sep. 30, 2011 Consumers Energy Company [Member] Supplemental Executive Retirement Plan [Member] | Sep. 30, 2011 Supplemental Executive Retirement Plan [Member] | Sep. 30, 2011 Consumers Energy Company [Member] Mutual Fund [Member] | Dec. 31, 2010 Consumers Energy Company [Member] Mutual Fund [Member] | Sep. 30, 2011 Mutual Fund [Member] | Dec. 31, 2010 Mutual Fund [Member] | Sep. 30, 2011 Consumers Energy Company [Member] State and Municipal Bonds [Member] | Dec. 31, 2010 Consumers Energy Company [Member] State and Municipal Bonds [Member] | Sep. 30, 2011 State and Municipal Bonds [Member] | Dec. 31, 2010 State and Municipal Bonds [Member] | Sep. 30, 2011 Consumers Energy Company [Member] CMS Energy Common Stock [Member] | Dec. 31, 2010 Consumers Energy Company [Member] CMS Energy Common Stock [Member] | Sep. 30, 2011 Consumers Energy Company [Member] | Dec. 31, 2010 Consumers Energy Company [Member]
Available for sale - cost |  |  |  |  | $ 57 | $ 39 | $ 88 | $ 62 |  |  |  |  | $ 7 | $ 8 |  |
Available for sale - unrealized gains |  |  |  |  |  |  |  |  |  |  | 1 |  | 24 | 26 |  |
Available for sale - unrealized losses |  |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |
Available for sale - fair value | 113 | 90 | 17 | 26 | 57 | 39 | 87 | 62 | 17 | 17 | 26 | 28 | 31 | 34 | 105 | 90
Available for sale State and municipal bonds - cost |  |  |  |  |  |  |  |  | 17 | 17 | 25 | 28 |  |  |  |
Held to maturity Debt securities - cost | 7 | 5 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Held to maturity Debt securities - unrealized gains | 1 | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Held to maturity Debt securities - unrealized losses |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Held to maturity Debt securities - fair value | 8 | 6 |  |  |  |  |  |  |  |  |  |  |  |  |  |
SERP Contributions |  |  | $ 20 | $ 27 |  |  |  |  |  |  |  |  |  |  |  |

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- Definition This element represents the portion of the balance sheet assertion valued at fair value by the entity whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission. This item represents Available-for-sale Securities which consist of all investments in certain debt and equity securities neither classified as trading or held-to-maturity securities. A debt security represents a creditor relationship with an enterprise. Debt securities include, among other items, US Treasury securities, US government securities, municipal securities, corporate bonds, convertible debt, commercial paper, and all securitized debt instruments. An equity security represents an ownership interest in an enterprise or the right to acquire or dispose of an ownership interest in an enterprise at fixed or determinable prices. Equity securities include, among other things, common stock, certain preferred stock, warrant rights, call options, and put options, but do not include convertible debt. An entity may opt to provide the reader with additional narrative text to better understand the nature of investments in debt and equity securities which are categorized as Available-for-sale. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 115 -Paragraph 12 -Subparagraph b -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 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=6379932&loc=d3e22054-111558 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Glossary Available-for-Sale Securities -URI http://asc.fasb.org/extlink&oid=6505594 + Details
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