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Consolidated Statements Of Comprehensive Income (Parenthetical) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Retirement benefits liability adjustments, tax expense (tax benefit)    $ 1    $ 1
Unrealized gain (loss) on investments, tax expense (tax benefit) 1    1   
Consumers Energy Company [Member]
       
Retirement benefits liability adjustments, tax expense (tax benefit)            
Unrealized gain (loss) on investments, tax expense (tax benefit) $ 1    $ (1) $ (1)