Earnings Per Share - CMS Energy (Basic and Diluted EPS Computation) (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended
Mar. 31, 2012	Mar. 31, 2011
Earnings Per Share - CMS Energy [Abstract]
Income from Continuing operations available to common stockholders basic and diluted	$ 60	$ 133
Weighted average shares basic	255.6	250.0
Add dilutive contingently convertible securities	10.4	10.7
Add dilutive Non vested awards and options	0.8	0.3
Add dilutive convertible debentures		0.7
Weighted average shares - diluted	266.8	261.7
Basic	$ 0.23	$ 0.53
Diluted	$ 0.22	$ 0.51

This element represents the income or loss from continuing operations attributable to the parent which may also be defined as revenue less expenses and taxes from ongoing operations before extraordinary items but after deduction of those portions of income or loss from continuing operations that are allocable to noncontrolling interests, if any.

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The amount of net income (loss) from continuing operations per each share of common stock or unit outstanding during the reporting period.

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- Definition

The amount of net income (loss) derived from continuing operations during the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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- Definition

Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of contingently issuable shares. Contingently issuable shares are those shares that are issuable for little or no cash contingent on certain conditions being met.

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- Definition

Nonvested equity-based payment awards that contain forfeitable rights to dividends or dividend equivalents (whether paid or unpaid) that result in incremental common shares.

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- Definition

The average number of shares or units issued and outstanding that are used in calculating diluted EPS or earnings per unit (EPU), determined based on the timing of issuance of shares or units in the period.

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- Definition

This element represents the weighted average total number of shares issued throughout the period including the first (beginning balance outstanding) and last (ending balance outstanding) day of the period before considering any reductions (for instance, shares held in treasury) to arrive at the weighted average number of shares outstanding. Weighted average relates to the portion of time within a reporting period that common shares have been issued and outstanding to the total time in that period. Such concept is used in determining the weighted average number of shares outstanding for purposes of calculating earnings per share (basic).

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