Fair Value Measurements (Assets And Liabilties Measured At Fair Value On A Recurring Basis) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Cash equivalents	$ 53	$ 109
Restricted cash equivalents	14	15
Nonqualified deferred compensation plan assets	5	4
Commodity contracts	3	3
Total	203	245
Nonqualified deferred compensation plan liabilities	5	4
Commodity contracts	4	7
Total	9	11
Consumers Energy Company [Member]
Cash equivalents		56
Restricted cash equivalents	13	14
Nonqualified deferred compensation plan assets	4	3
Commodity contracts	2	2
Total	137	185
Nonqualified deferred compensation plan liabilities	4	3
Total	4	3
Fair Value, Inputs, Level 1 [Member]
Cash equivalents	53	109
Restricted cash equivalents	14	15
Nonqualified deferred compensation plan assets	5	4
Commodity contracts		1
Total	200	243
Nonqualified deferred compensation plan liabilities	5	4
Total	5	4
Fair Value, Inputs, Level 1 [Member] \| Consumers Energy Company [Member]
Cash equivalents		56
Restricted cash equivalents	13	14
Nonqualified deferred compensation plan assets	4	3
Total	135	183
Nonqualified deferred compensation plan liabilities	4	3
Total	4	3
Fair Value, Inputs, Level 2 [Member]
Commodity contracts	3	3
Total	3	3
Fair Value, Inputs, Level 3 [Member]
Commodity contracts	3	2
Total	3	2
Commodity contracts	1	4
Total	1	4
Fair Value, Inputs, Level 3 [Member] \| Consumers Energy Company [Member]
Commodity contracts	2	2
Total	2	2
Supplemental Executive Retirement Plan [Member] \| Cash Equivalents [Member]
Available for sale - Fair Value	2	1
Supplemental Executive Retirement Plan [Member] \| Cash Equivalents [Member] \| Consumers Energy Company [Member]
Available for sale - Fair Value	1	1
Supplemental Executive Retirement Plan [Member] \| Mutual Fund [Member]
Available for sale - Fair Value	126	113
Supplemental Executive Retirement Plan [Member] \| Mutual Fund [Member] \| Consumers Energy Company [Member]
Available for sale - Fair Value	85	74
Supplemental Executive Retirement Plan [Member] \| Fair Value, Inputs, Level 1 [Member] \| Cash Equivalents [Member]
Available for sale - Fair Value	2	1
Supplemental Executive Retirement Plan [Member] \| Fair Value, Inputs, Level 1 [Member] \| Cash Equivalents [Member] \| Consumers Energy Company [Member]
Available for sale - Fair Value	1	1
Supplemental Executive Retirement Plan [Member] \| Fair Value, Inputs, Level 1 [Member] \| Mutual Fund [Member]
Available for sale - Fair Value	126	113
Supplemental Executive Retirement Plan [Member] \| Fair Value, Inputs, Level 1 [Member] \| Mutual Fund [Member] \| Consumers Energy Company [Member]
Available for sale - Fair Value	85	74
CMS Energy Common Stock [Member] \| Consumers Energy Company [Member]
Available for sale - Fair Value	32	35
CMS Energy Common Stock [Member] \| Fair Value, Inputs, Level 1 [Member] \| Consumers Energy Company [Member]
Available for sale - Fair Value	$ 32	$ 35

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 includes currency on hand as well as demand deposits with banks or financial institutions. It also includes other kinds of accounts that have the general characteristics of demand deposits in that the Company may deposit additional funds at any time and also effectively may withdraw funds at any time without prior notice or penalty. Cash equivalents, excluding items classified as marketable securities, include short-term, highly liquid investments that are both readily convertible to known amounts of cash, and so near their maturity that they present minimal risk of changes in value because of changes in interest rates. Generally, only investments with original maturities of three months or less qualify under that definition. Original maturity means original maturity to the entity holding the investment. For example, both a three-month US Treasury bill and a three-year Treasury note purchased three months from maturity qualify as cash equivalents. However, a Treasury note purchased three years ago does not become a cash equivalent when its remaining maturity is three months. This item represents restricted cash equivalents only.

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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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- Definition

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- Definition

Derivative instrument obligations meeting the definition of a liability which the Company is a party to as of the balance sheet date.

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