Financial Instruments (Schedule Of Cost/Carrying Amounts And Fair Values Of Financial Instruments) (Details) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities held to maturity, Cost or Carrying Amount $ 8 $ 7
Securities held to maturity - Fair Value 9 7
Long-term debt, Cost or Carrying Amount 7,199 [1] 7,073 [1]
Long-term debt, Fair Value 8,251 [1] 8,025 [1]
Notes receivable 28 49
Current portion of long-term debt 488 1,033
Consumers Energy Company [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt, Cost or Carrying Amount 4,348 [2] 4,326 [2]
Long-term debt, Fair Value 4,987 [2] 4,882 [2]
Notes receivable   23
Current portion of long-term debt 40 339
EnerBank USA [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Notes receivable, Carrying Amount 513 [3] 480 [3]
Notes receivable, Fair Value 541 [3] 504 [3]
Notes receivable 27 19
Fair Value, Inputs, Level 2 [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Securities held to maturity - Fair Value 9  
Long-term debt, Fair Value 8,251 [1]  
Fair Value, Inputs, Level 2 [Member] | Consumers Energy Company [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Long-term debt, Fair Value 4,987 [2]  
Fair Value, Inputs, Level 3 [Member] | EnerBank USA [Member]
    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Notes receivable, Fair Value $ 541 [3]  
[1] Includes current portion of long-term debt of $488 million at September 30, 2012 and $1,033 million at December 31, 2011.
[2] Includes current portion of long-term debt of $40 million at September 30, 2012 and $339 million at December 31, 2011.
[3] In Millions September 30, 2012December 31, 2011Carrying Fair ValueCarrying Amount Total Level 1 Level 2 Level 3 Amount Fair Value CMS Energy, including ConsumersSecurities held to maturity$ 8$ 9$ -$ 9$ -$ 7$ 7Notes receivable1 513 541 - - 541 480 504Long-term debt2 7,199 8,251 - 8,251 - 7,073 8,025ConsumersLong-term debt3$ 4,348$ 4,987$ -$ 4,987$ -$ 4,326$ 4,882Includes current portion of notes receivable of $27 million at September 30, 2012 and $19 million at December 31, 2011.