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Income Taxes (Details)
 3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
U.S. federal income tax rate 35.00% 35.00%
State and local income taxes, net of federal benefit 4.20% 3.40%
Medicare Part D exempt income 0.10% (1.20%)
Income tax credit amortization (0.50%) (0.60%)
Other, net   0.10%
Effective income tax rate 38.80% 36.70%
Consumers Energy Company [Member]
    
U.S. federal income tax rate 35.00% 35.00%
State and local income taxes, net of federal benefit 5.60% 3.50%
Medicare Part D exempt income 0.50% (0.90%)
Plant basis differences (0.80%) (0.20%)
Income tax credit amortization (0.40%) (0.40%)
Other, net 0.70% 0.20%
Effective income tax rate 40.60% 37.20%