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Financial Instruments (Narrative) (Details) (USD $)
In Millions, unless otherwise specified
1 Months Ended 9 Months Ended
Jan. 31, 2012
Consumers Energy Company [Member]
Jan. 31, 2011
Consumers Energy Company [Member]
Sep. 30, 2012
Consumers Energy Company [Member]
Dec. 31, 2011
Consumers Energy Company [Member]
Sep. 30, 2012
Supplemental Executive Retirement Plan [Member]
Sep. 30, 2012
Supplemental Executive Retirement Plan [Member]
Consumers Energy Company [Member]
Financial Instruments [Line Items]            
Portion of long-term debt supported by third-party credit enhancements     $ 103 $ 103    
SERP Contributions         13 9
Gain on donation of CMS Energy common stock $ 5 $ 4