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Retirement Benefits (Schedule of Net Benefit Costs) (Details) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Mar. 31, 2012
Mar. 31, 2011
Other Postretirement Benefit Plans, Defined Benefit [Member]
    
Service cost $ 8 $ 7
Interest expense 21 19
Expected return on plan assets (17) (17)
Amortization of Net loss 11 8
Amortization of Prior service cost (5) (5)
Net periodic cost 18 12
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
    
Service cost 8 6
Interest expense 20 18
Expected return on plan assets (15) (15)
Amortization of Net loss 11 8
Amortization of Prior service cost (5) (5)
Net periodic cost 19 12
Pension Plans, Defined Benefit [Member]
    
Service cost 12 12
Interest expense 25 25
Expected return on plan assets (31) (28)
Amortization of Net loss 19 16
Amortization of Prior service cost 1 1
Net periodic cost 26 26
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
    
Service cost 12 12
Interest expense 24 24
Expected return on plan assets (30) (27)
Amortization of Net loss 18 15
Amortization of Prior service cost 1 1
Net periodic cost $ 25 $ 25