Financial Instruments (Schedule of Cost of Carrying Amounts and Fair Value of Long-Term Financial Instruments) (Details) (USD $)
In Millions | Sep. 30, 2011 | Dec. 31, 2010 |
|---|
| Securities held to maturity, Cost or Carrying Amount | $ 7 | | $ 5 | |
| Securities held to maturity - fair value | 8 | | 6 | |
| Securities available for sale, Cost or Carrying Amount | 113 | | 90 | |
| Securities available for sale, fair value | 113 | | 90 | |
| Total unsecured consumer installment loans, net of an allowance for loan losses | 439 | [1] | 386 | [1] |
| Notes receivable, Fair Value | 459 | [1] | 407 | [1] |
| Long-term debt, Cost or Carrying Amount | 7,153 | [2] | 7,174 | [2] |
| Long-term Debt, Fair Value | 8,093 | [2] | 7,861 | [2] |
| Notes receivable | 62 | | 70 | |
| Current portion of long-term debt | 1,116 | | 726 | |
| Portion of long-term debt supported by third-party credit enhancements | 103 | | 103 | |
Consumers Energy Company [Member] | | | | |
| Securities available for sale, Cost or Carrying Amount | 81 | | 64 | |
| Securities available for sale, fair value | 105 | | 90 | |
| Long-term debt, Cost or Carrying Amount | 4,335 | [3] | 4,525 | [3] |
| Long-term Debt, Fair Value | 4,901 | [3] | 4,891 | [3] |
| Notes receivable | 43 | | 55 | |
| Current portion of long-term debt | 338 | | 37 | |
| Portion of long-term debt supported by third-party credit enhancements | 103 | | 103 | |
EnerBank USA [Member] | | | | |
| Total unsecured consumer installment loans, net of an allowance for loan losses | 439 | | 386 | |
| Notes receivable | $ 16 | | $ 11 | |
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