Income Taxes (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended9 Months Ended
Jun. 30, 2011
Sep. 30, 2011
Sep. 30, 2010
U.S. federal income tax rate 35.00%35.00%
MCIT law change, net of federal expense (5.90%) 
State and local income taxes, net of federal benefit 3.40%4.00%
Medicare Part D exempt income, net of law change (0.90%)(0.90%)
Income tax credit amortization (0.60%)(0.60%)
Other, net 0.10%0.90%
Effective income tax rate 31.10%38.40%
MCIT rate6.00%  
Expense Reversal on Previous Benefit from the Medicare Subsidy  $ 3
Consumers Energy Company [Member] | Michigan Single Business Tax [Member]
   
Reversal of net deferred tax assets related to MCIT law change134  
Recognition of regulatory asset related to MCIT law change134  
Michigan Single Business Tax [Member]
   
Elimination of deferred tax liabilities related to MCIT law change32  
Recognition of tax benefit related to MCIT law change 32  
Consumers Energy Company [Member]
   
U.S. federal income tax rate 35.00%35.00%
State and local income taxes, net of federal benefit 3.10%3.50%
Medicare Part D exempt income, net of law change (0.80%)(1.30%)
Plant basis differences 0.20%0.30%
Income tax credit amortization (0.50%)(0.50%)
Other, net (0.30%)(0.20%)
Effective income tax rate 36.70%36.80%
MCIT rate6.00%