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Financings And Capitalization (Contingently Convertible Securities) (Details) (USD $) |  |  | 12 Months Ended
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Dec. 31, 2012 CMS Energy [Member] | Dec. 31, 2011 CMS Energy [Member] | Dec. 31, 2012 Senior Notes 5.5% Due June 2029 [Member] | Dec. 31, 2012 2.875% Convertible Senior Notes Due 2024 [Member] | Dec. 31, 2012 2.875% Convertible Senior Notes Due 2024 [Member] January Conversion [Member] | Dec. 31, 2012 2.875% Convertible Senior Notes Due 2024 [Member] April Conversion [Member]
Maturity Date |  |  | 2029 | 2024 |  |
Outstanding |  |  | $ 172,000,000 |  |  |
Adjusted Conversion Price |  |  | $ 13.94 |  |  |
Adjusted Trigger Price |  |  | $ 18.12 |  |  |
Number of trading days, adjusted trigger price contingencies were met |  |  | 20 days |  |  |
Number of trading days |  |  | 30 days |  |  |
Trigger Price As Percent Of Conversion Price |  |  | 130.00% |  |  |
Senior Notes Interest Rate |  |  | 5.50% | 2.875% |  |
Conversion Date |  |  |  |  | January 2012 | April 2012
Principal Converted |  |  |  |  | 73,000,000 | 153,000,000
Conversion Value per share of principal |  |  |  |  | 1,738.99 | 1,774.98
Debt issuance, principal | 2,377,000,000 | 2,273,000,000 |  |  |  |
Common Stock Issued on Settlement |  |  |  |  | 2,464,138 | 5,381,349
Cash Paid on Settlement |  |  |  |  | $ 73,000,000 | $ 153,000,000

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