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Retirement Benefits (Narrative) (Details) (USD $) In Millions, unless otherwise specified | 3 Months Ended | 9 Months Ended |  |  | 9 Months Ended | 3 Months Ended | 9 Months Ended |  |  | 9 Months Ended
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Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 | Sep. 30, 2012 | Sep. 30, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member] | Dec. 31, 2012 Other Postretirement Benefit Plans, Defined Benefit [Member] | Sep. 30, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member] Remeasurement As Of July 1, 2013 [Member] | Sep. 30, 2013 Consumers Energy Company [Member] | Sep. 30, 2012 Consumers Energy Company [Member] | Sep. 30, 2013 Consumers Energy Company [Member] | Sep. 30, 2012 Consumers Energy Company [Member] | Sep. 30, 2013 Consumers Energy Company [Member] Other Postretirement Benefit Plans, Defined Benefit [Member] | Dec. 31, 2012 Consumers Energy Company [Member] Other Postretirement Benefit Plans, Defined Benefit [Member] | Sep. 30, 2013 Consumers Energy Company [Member] Other Postretirement Benefit Plans, Defined Benefit [Member] Remeasurement As Of July 1, 2013 [Member]
Defined Benefit Plan Disclosure [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Discount rate |  |  |  |  | 5.10% | 4.40% |  |  |  |  |  | 5.10% | 4.40% |
Deacrease in liability |  |  |  |  |  |  | $ 638 |  |  |  |  |  |  | $ 614
Regulatory asset decrease |  |  |  |  |  |  | 580 |  |  |  |  |  |  | 580
Increase in regulatory liabilities |  |  |  |  |  |  | 34 |  |  |  |  |  |  | 34
Decrease in AOCL |  |  |  |  |  |  | 24 |  |  |  |  |  |  |
Decrease in expected cost |  |  |  |  |  |  | 48 |  |  |  |  |  |  | 46
Decrease deferred tax assets |  |  |  |  |  |  | 148 |  |  |  |  |  |  | 144
Reduced regulatory income tax liabilities |  |  |  |  |  |  | 144 |  |  |  |  |  |  | 144
Increased income tax expense | $ 91 | $ 98 | $ 240 | $ 203 |  |  | $ 4 | $ 102 | $ 107 | $ 272 | $ 239 |  |  |

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