Financial Instruments (Schedule of Contractual Maturity Securities) (Details) (USD $) In Millions	Jun. 30, 2011	Dec. 31, 2010
Available for sale - fair value	$ 116	$ 90
Consumers Energy Company [Member] \| Supplemental Executive Retirement Plan [Member]
Available for sale state and municipal bonds - Due within one year	1
Available for sale state and municipal bonds - Due after one year through five years	7
Available for sale state and municipal bonds - Due after five year through ten years	7
Available for sale state and municipal bonds - Due after ten years	2
Available for sale - fair value	17
Supplemental Executive Retirement Plan [Member]
Available for sale state and municipal bonds - Due within one year	1
Available for sale state and municipal bonds - Due after one year through five years	10
Available for sale state and municipal bonds - Due after five year through ten years	12
Available for sale state and municipal bonds - Due after ten years	3
Available for sale - fair value	26
Consumers Energy Company [Member]
Available for sale - fair value	$ 107	$ 90

This item represents the fair value of debt securities which are expected to mature after five years and through ten years from the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

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This item represents the fair value of debt securities which are expected to mature after one year and through five years from the balance sheet date and which are categorized neither as held-to-maturity nor trading securities.

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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.

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