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Retirement Benefits (Schedule of Allocation of Plan Assets) (Details) (USD $) In Millions, unless otherwise specified | Dec. 31, 2012 | Dec. 31, 2011
--- | --- | ---
Pension Plans, Defined Benefit [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | $ 1,727 |  | $ 1,626 |
Investment grade bonds included in the corporate debt investments category of the plan trust assets | 68.00% |  | 69.00% |
High-yield bonds included in the corporate debt investments category of the Plan trust assets | 32.00% |  | 31.00% |
U.S. equity securities included in the Pooled funds category of the Plan trust assets | 51.00% |  | 53.00% |
Foreign equity securities included in the Pooled funds category of the Plan trust assets | 26.00% |  | 22.00% |
Foreign fixed-income securities included in the Pooled funds category of the Plan trust assets | 14.00% |  | 16.00% |
U.S. fixed-income securities included in the Pooled funds category of the Plan trust assets | 4.00% |  | 5.00% |
Alternative investments included in the Pooled funds category of the Plan trust assets | 5.00% |  | 4.00% |
Pension Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 352 |  | 498 |
Pension Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 1,375 |  | 1,128 |
Pension Plans, Defined Benefit [Member] | Cash and Short-Term Investments [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 33 | [1] | 241 |
Pension Plans, Defined Benefit [Member] | Cash and Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 33 | [1] | 241 |
Pension Plans, Defined Benefit [Member] | U.S. Government and Agencies Securities [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 26 | [2] | 24 | [2]
Pension Plans, Defined Benefit [Member] | U.S. Government and Agencies Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 26 | [2] | 24 | [2]
Pension Plans, Defined Benefit [Member] | Corporate Debt [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 277 | [3] | 236 |
Pension Plans, Defined Benefit [Member] | Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 277 | [3] | 236 |
Pension Plans, Defined Benefit [Member] | State and Municipal Bonds [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 8 | [4] | 10 | [4]
Pension Plans, Defined Benefit [Member] | State and Municipal Bonds [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 8 | [4] | 10 | [4]
Pension Plans, Defined Benefit [Member] | Foreign Corporate Debt [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 27 | [5] | 23 |
Pension Plans, Defined Benefit [Member] | Foreign Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 27 | [5] | 23 |
Pension Plans, Defined Benefit [Member] | Mutual Funds [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 319 | [6] | 257 | [6]
Pension Plans, Defined Benefit [Member] | Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 319 | [6] | 257 | [6]
Pension Plans, Defined Benefit [Member] | Pooled Funds [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 1,037 | [7] | 835 |
Pension Plans, Defined Benefit [Member] | Pooled Funds [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 1,037 | [7] | 835 |
Other Postretirement Benefit Plans, Defined Benefit [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 1,047 |  | 924 |
Investment grade bonds included in the corporate debt investments category of the plan trust assets | 68.00% |  | 69.00% |
High-yield bonds included in the corporate debt investments category of the Plan trust assets | 32.00% |  | 31.00% |
U.S. equity securities included in the Pooled funds category of the Plan trust assets | 65.00% |  | 88.00% |
Foreign equity securities included in the Pooled funds category of the Plan trust assets | 21.00% |  | 6.00% |
Foreign fixed-income securities included in the Pooled funds category of the Plan trust assets | 9.00% |  | 4.00% |
U.S. fixed-income securities included in the Pooled funds category of the Plan trust assets | 3.00% |  | 1.00% |
Alternative investments included in the Pooled funds category of the Plan trust assets | 2.00% |  | 1.00% |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 978 |  | 861 |
Investment grade bonds included in the corporate debt investments category of the plan trust assets | 68.00% |  | 69.00% |
High-yield bonds included in the corporate debt investments category of the Plan trust assets | 32.00% |  | 31.00% |
U.S. equity securities included in the Pooled funds category of the Plan trust assets | 65.00% |  | 88.00% |
Foreign equity securities included in the Pooled funds category of the Plan trust assets | 21.00% |  | 6.00% |
Foreign fixed-income securities included in the Pooled funds category of the Plan trust assets | 9.00% |  | 4.00% |
U.S. fixed-income securities included in the Pooled funds category of the Plan trust assets | 3.00% |  | 1.00% |
Alternative investments included in the Pooled funds category of the Plan trust assets | 2.00% |  | 1.00% |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 493 |  | 208 |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 1 [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 462 |  | 194 |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 554 |  | 716 |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 516 |  | 667 |
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and Short-Term Investments [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 118 |  | 64 | [1]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and Short-Term Investments [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 111 |  | 60 | [1]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 118 |  | 64 | [1]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Cash and Short-Term Investments [Member] | Fair Value, Inputs, Level 1 [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 111 |  | 60 | [1]
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Government and Agencies Securities [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 4 | [2] | 203 | [2]
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Government and Agencies Securities [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 3 | [2] | 189 | [2]
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Government and Agencies Securities [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 4 | [2] | 203 | [2]
Other Postretirement Benefit Plans, Defined Benefit [Member] | U.S. Government and Agencies Securities [Member] | Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 3 | [2] | 189 | [2]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Corporate Debt [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 38 | [3] | 28 | [3]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Corporate Debt [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 35 | [3] | 26 | [3]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 38 | [3] | 28 | [3]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 35 | [3] | 26 | [3]
Other Postretirement Benefit Plans, Defined Benefit [Member] | State and Municipal Bonds [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 1 | [4] | 71 | [4]
Other Postretirement Benefit Plans, Defined Benefit [Member] | State and Municipal Bonds [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 1 | [4] | 66 | [4]
Other Postretirement Benefit Plans, Defined Benefit [Member] | State and Municipal Bonds [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 1 | [4] | 71 | [4]
Other Postretirement Benefit Plans, Defined Benefit [Member] | State and Municipal Bonds [Member] | Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 1 | [4] | 66 | [4]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Foreign Corporate Debt [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 4 |  | 3 | [5]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Foreign Corporate Debt [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 3 |  | 3 | [5]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Foreign Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 4 |  | 3 | [5]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Foreign Corporate Debt [Member] | Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 3 |  | 3 | [5]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common Stocks [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 75 | [8] | 113 | [8]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common Stocks [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 70 | [8] | 105 | [8]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common Stocks [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 75 | [8] | 113 | [8]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Common Stocks [Member] | Fair Value, Inputs, Level 1 [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 70 | [8] | 105 | [8]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Mutual Funds [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 300 | [6] | 31 | [6]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Mutual Funds [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 281 | [6] | 29 | [6]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 300 | [6] | 31 | [6]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Mutual Funds [Member] | Fair Value, Inputs, Level 1 [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 281 | [6] | 29 | [6]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Pooled Funds [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 507 |  | 411 | [9]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Pooled Funds [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 474 |  | 383 | [9]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Pooled Funds [Member] | Fair Value, Inputs, Level 2 [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | 507 |  | 411 | [9]
Other Postretirement Benefit Plans, Defined Benefit [Member] | Pooled Funds [Member] | Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member] |  |  |  |
Defined Benefit Plan, Assets for Plan Benefits | $ 474 |  | $ 383 | [9]
[1] | Cash and short-term investments consist of money market funds with daily liquidity.
[2] | U.S. government and agencies securities consist of U.S. Treasury notes and other debt securities backed by the U.S. government and related agencies. These securities were valued based on quoted market prices.
[3] | At December 31, 2012, corporate debt investments in the OPEB plan comprised investment grade bonds (68 percent) and non-investment grade, high-yield bonds (32 percent) of U.S. issuers from diverse industries. At December 31, 2011, corporate debt investments in the OPEB plan comprised investment grade bonds (69 percent) and non-investment grade, high-yield bonds (31 percent) of U.S. issuers from diverse industries. These securities are valued based on quoted market prices, when available, or yields presently available on comparable securities of issuers with similar credit ratings.
[4] | State and municipal bonds were valued using a matrix-pricing model that incorporates Level 2 market-based information. The fair value of the bonds was derived from various observable inputs, including benchmark yields, reported securities trades, broker/dealer quotes, bond ratings, and general information on market movements for investment grade state and municipal securities normally considered by market participants when pricing such debt securities.
[5] | Foreign corporate debt securities were valued based on quoted market prices, when available, or on yields available on comparable securities of issuers with similar credit ratings.
[6] | Mutual funds represent shares in registered investment companies that are priced based on the daily quoted NAVs that are publicly available and are the basis for transactions to buy or sell shares in the funds.
[7] | Pooled funds in the Pension Plan include both common and collective trust funds as well as special funds that contain only employee benefit plan assets from two or more unrelated benefit plans. At December 31, 2012, these funds comprised investments in U.S. equity securities (51 percent), foreign equity securities (26 percent), foreign fixed-income securities (14 percent), U.S. fixed-income securities (four percent), and alternative investments (five percent). At December 31, 2011, these funds comprised investments in U.S. equity securities (53 percent), foreign equity securities (22 percent), foreign fixed-income securities (16 percent), U.S. fixed-income securities (five percent), and alternative investments (four percent). These investments were valued at the quoted NAV provided by the fund managers that is the basis for transactions to buy or sell shares in the funds.
[8] | Common stocks in the OPEB plan consist of equity securities with low transaction costs that were actively managed and tracked by the S&P&#160;500 Index. These securities were valued at their quoted closing prices.
[9] | Pooled funds in the OPEB plan include both common and collective trust funds as well as special funds that contain only employee benefit plan assets from two or more unrelated benefit plans. At December 31, 2012, these funds comprised investments in U.S. equity securities (65 percent), foreign equity securities (21 percent), foreign fixed-income securities (nine percent), U.S. fixed-income securities (three percent), and alternative investments (two percent). At December 31, 2011, these funds comprised investments in U.S. equity securities (88 percent), foreign equity securities (six percent), foreign fixed-income securities (four percent), U.S. fixed-income securities (one percent), and alternative investments (one percent). These investments are valued at the quoted NAV provided by the fund managers that is the basis for transactions to buy or sell shares in the funds.

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- Definition This represents the entire assets recognized in the balance sheet that are associated with the defined benefit plans. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 45 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=21915240&loc=d3e1703-114919 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.10) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.10) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Statement of Financial Accounting Standard (FAS) -Number 132R -Paragraph 5 -Subparagraph c -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 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 20 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=21915506&loc=d3e1928-114920 + Details
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