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Financings And Capitalization (Major Long-Term Debt Transactions) (Details) (USD $) |  |  |  |  |  |  |  |  | 1 Months Ended |  | 12 Months Ended |  | 12 Months Ended |  |  |  | 12 Months Ended
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Dec. 31, 2012 | Dec. 31, 2012 CMS Energy [Member] | Dec. 31, 2011 CMS Energy [Member] | Dec. 31, 2012 Consumers Energy Company [Member] | Dec. 31, 2012 First Mortgage Bonds [Member] Consumers Energy Company [Member] | Dec. 31, 2011 First Mortgage Bonds [Member] Consumers Energy Company [Member] | Feb. 28, 2013 Term Loan Facility Due December 2016 [Member] | Dec. 31, 2012 Term Loan Facility Due December 2016 [Member] | Jun. 30, 2012 Term Loan Facility Due March 2013 [Member] Consumers Energy Company [Member] | Aug. 31, 2012 Tax Exempt Bonds Variable Due April 2018 [Member] Consumers Energy Company [Member] | Aug. 31, 2012 Tax Exempt Bonds Variable Due April 2035 [Member] Consumers Energy Company [Member] | Aug. 31, 2012 Tax Exempt Bonds Variable [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Issuances [Member] | Dec. 31, 2012 Debt Issuances [Member] CMS Energy [Member] | Dec. 31, 2012 Debt Issuances [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Issuances [Member] First Mortgage Bonds 2.85% Due May 2022 [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Issuances [Member] First Mortgage Bonds 3.19% Due 2024 [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Issuances [Member] First Mortgage Bonds 3.39% Due 2027 [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Issuances [Member] First Mortgage Bonds 4.31% Due 2042 [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Issuances [Member] Senior Notes 5.05% Due March 2022 [Member] CMS Energy [Member] | Dec. 31, 2012 Debt Issuances [Member] Term Loan Facility Due December 2016 [Member] CMS Energy [Member] | Feb. 28, 2013 Debt Issuances [Member] Term Loan Facility Due December 2016 [Member] CMS Energy [Member] | Dec. 31, 2012 Debt Issuances [Member] Term Loan Facility Due March 2013 [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Issuances [Member] Tax Exempt Bonds Variable Due April 2018 [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Issuances [Member] Tax Exempt Bonds Variable Due April 2035 [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Retirements [Member] | Dec. 31, 2012 Debt Retirements [Member] CMS Energy [Member] | Dec. 31, 2012 Debt Retirements [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Retirements [Member] First Mortgage Bonds 5% Due February 2012 [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Retirements [Member] First Mortgage Bonds 5.375% Due April 2013 [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Retirements [Member] Term Loan Facility Due March 2013 [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Retirements [Member] 2.875% Convertible Senior Notes Due 2024 [Member] CMS Energy [Member] | Dec. 31, 2012 Debt Retirements [Member] Tax Exempt Bonds Variable Due April 2018 [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Retirements [Member] Tax Exempt Bonds Variable Due April 2035 [Member] Consumers Energy Company [Member] | Dec. 31, 2012 Debt Retirements [Member] Senior Notes Variable Rate Due January 2013 [Member] CMS Energy [Member] | Dec. 31, 2012 Debt Retirements [Member] Trust Preferred Securities [Member] | Dec. 31, 2011 Debt Retirements [Member] Trust Preferred Securities [Member] | Dec. 31, 2010 Debt Retirements [Member] Trust Preferred Securities [Member] | Dec. 31, 2012 Debt Retirements [Member] Trust Preferred Securities [Member] CMS Energy [Member]
Debt issuance, principal |  | $ 2,377,000,000 | $ 2,273,000,000 |  | $ 3,925,000,000 | [1] | $ 3,875,000,000 | [1] |  | $ 180,000,000 |  | $ 68,000,000 | $ 35,000,000 |  | $ 1,658,000,000 | $ 480,000,000 | $ 1,178,000,000 | $ 375,000,000 | $ 52,000,000 | $ 35,000,000 | $ 263,000,000 | $ 300,000,000 | $ 180,000,000 | [2] |  | $ 350,000,000 | [3] | $ 68,000,000 | [4] | $ 35,000,000 | [4] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt retirement, principal |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 1,533,000,000 | 405,000,000 | 1,128,000,000 | 300,000,000 | 375,000,000 | 350,000,000 | [3] | 226,000,000 | [5] | 68,000,000 | [4] | 35,000,000 | [4] | 150,000,000 |  |  |  | 29,000,000
Interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2.85% | 3.19% | 3.39% | 4.31% | 5.05% |  |  |  |  |  |  |  |  |  |  |  |  | 5.00% | 5.375% |  |  | 2.875% | [5] |  |  |  |  |  |  |  |  | 7.75%
Interest rate (variable) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | variable | [2] |  | variable | [3] | variable | [4] | variable | [4] |  |  |  |  |  | variable | [3] |  |  | variable | [4] | variable | [4] | variable |  |  |  |
Debt issuance date |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | May 2012 | December 2012 | December 2012 | December 2012 | March 2012 | February 2012 and July 2012 | [2] |  | July 2012 | [3] | August 2012 | [4] | August 2012 | [4] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt retirement date |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | February 2012 | May 2012 | December 2012 | [3] | January 2012 and April 2012 | [5] | August 2012 | [4] | August 2012 | [4] | July 2012 | February 2012 | February 2012 | February 2012 | February 2012
Maturity date |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | May 2022 | December 2024 | December 2027 | December 2042 | March 2022 | December 2016 | [2] |  | March 2013 | [3] | April 2018 | [4] | April 2035 | [4] |  |  |  | February 2012 | April 2013 | March 2013 | [3] | December 2024 | [5] | April 2018 | [4] | April 2035 | [4] | January 2013 |  |  |  | July 2027
Maximum allowed short term securities outstanding under FERC Authorization | 500,000,000 |  |  | 500,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Short-term securites, remaining availability under FERC Authorization | 500,000,000 |  |  | 500,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum allowed long-term securites outstanding under FERC Authorization | 1,900,000,000 |  |  | 1,900,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Long-term securities, remaining availability under FERC Authorization | 1,600,000,000 |  |  | 1,600,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt retirement face amount |  |  |  |  |  |  |  |  |  |  |  |  |  | 103,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate terms |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | LIBOR plus 2.50 percent |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
LIBOR plus |  |  |  |  |  |  |  |  | 2.00% | 2.50% |  |  |  |  |  |  |  |  |  |  |  |  | 2.50% |  | 2.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Interest rate at period end |  |  |  |  |  |  |  |  |  | 2.71% |  |  |  |  |  |  |  |  |  |  |  |  | 2.71% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Amount of Facility |  |  |  |  |  |  |  |  |  |  | $ 375,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
[1] | The weighted-average interest rate for Consumers’ FMBs was 5.19 percent at December 31, 2012 and 5.52 percent at December 31, 2011.
[2] | Outstanding borrowings bear interest at an annual interest rate of LIBOR plus 2.50 percent (2.71 percent at December 31, 2012). CMS Energy used these proceeds to retire the 7.75 percent Trust Preferred Securities and floating-rate senior notes due January 2013. In February 2013, the term loan facility was amended, reducing the annual interest rate to LIBOR plus 2.00 percent.
[3] | In June 2012, Consumers entered into a short-term credit agreement permitting Consumers to borrow up to $375 million. This agreement was terminated using proceeds from FMBs issued in December 2012.
[4] | In August 2012, Consumers utilized the Michigan Strategic Fund for the issuance of $68 million and $35 million of tax-exempt Michigan Strategic Fund revenue bonds. The bonds, which are backed by letters of credit and collateralized by Consumers’ FMBs, are subject to optional tender by the holders that would result in remarketing. Consumers used the proceeds to redeem $103 million of tax-exempt bonds in August 2012.
[5] | CMS Energy’s contingently convertible notes. See the “Contingently Convertible Securities” section in this Note for further discussion of the conversions.

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