Financial Instruments (Tables)
9 Months Ended
Sep. 30, 2011
Schedule of Cost/Carrying Amounts and Fair Values of Long-Term Financial Instruments
In Millions
    September 30, 2011     December 31, 2010
    Cost or             Cost or        
    Carrying             Carrying        
    Amount     Fair Value     Amount     Fair Value  
 
CMS Energy, including Consumers
                                
Securities held to maturity
   $ 7     $ 8     $ 5     $ 6  
Securities available for sale
     113       113       90       90  
Notes receivable1
     439       459       386       407  
Long-term debt2
     7,153       8,093       7,174       7,861  
 
Consumers
                                
Securities available for sale
   $ 81     $ 105     $ 64     $ 90  
Long-term debt3
     4,335       4,901       4,525       4,891  
 
1    Includes current portion of notes receivable of $16 million at September 30, 2011 and $11 million at December 31, 2010.
2    Includes current portion of long-term debt of $1,116 million at September 30, 2011 and $726 million at December 31, 2010.
3    Includes current portion of long-term debt of $338 million at September 30, 2011 and $37 million at December 31, 2010.
Schedule of Investment Securities
                                                                 
In Millions
    September 30, 2011     December 31, 2010
            Unrealized     Unrealized                     Unrealized     Unrealized        
    Cost     Gains     Losses     Fair Value     Cost     Gains     Losses     Fair Value  
   
CMS Energy, including consumers                                                
Available for sale
                                                                
SERP
                                                                
Mutual fund
   $ 88     $ —     $ 1     $ 87     $ 62     $ —     $ —     $ 62  
State and municipal bonds
     25       1       —       26       28       —       —       28  
Held to maturity
                                                                
Debt securities
     7       1       —       8       5       1       —       6  
   
Consumers
                                                                
Available for sale
                                                                
SERP
                                                                
Mutual fund
   $ 57     $ —     $ —     $ 57     $ 39     $ —     $ —     $ 39  
State and municipal bonds
     17       —       —       17       17       —       —       17  
CMS Energy common stock
     7       24       —       31       8       26       —       34  
 
Schedule of Proceeds and Gross Realized Gains/Losses from Sales of Available-for-Sale Securities
In Millions
    Three months ended   Nine months ended
September 30   2011   2010   2011   2010
 
CMS Energy, including Consumers
                            
Proceeds from sales of investment securities1
  $ 1    
$
—
   $ 2     $ 1  
 
Consumers
                            
Proceeds from sales of investment securities1
  $ 1    
$
—
   $ 1     $ —  
 
1     All of the proceeds related to sales of investments that were held within the SERP and classified as available for sale. Realized losses on these sales were less than $1 million for both CMS Energy and Consumers during each period.
Schedule of Contractual Maturities of Investments
                 
In Millions
    CMS Energy,        
    including Consumers     Consumers  
 
Due one year or less
   $ 2     $ 1  
Due after one year through five years
     9       6  
Due after five years through ten years
     12       8  
Due after ten years
     3       2  
 
Total
   $ 26     $ 17  
Consumers Energy Company [Member]
 
Schedule of Cost/Carrying Amounts and Fair Values of Long-Term Financial Instruments
In Millions  
    September 30, 2011     December 31, 2010  
    Cost or             Cost or        
    Carrying             Carrying        
    Amount     Fair Value     Amount     Fair Value  
 
CMS Energy, including Consumers
                                
Securities held to maturity
   $ 7     $ 7     $ 5     $ 6  
Securities available for sale
     113       113       90       90  
Notes receivable1
     439       460       386       407  
Long-term debt2
     7,153       8,093       7,174       7,861  
 
Consumers
                                
Securities available for sale
   $ 81     $ 105     $ 64     $ 90  
Long-term debt3
     4,336       4,901       4,525       4,891  
 
1   Includes current portion of notes receivable of $15 million at September 30, 2011 and $11 million at December 31, 2010.
 
2   Includes current portion of long-term debt of $1,116 million at September 30, 2011 and $726 million at December 31, 2010.
 
3   Includes current portion of long-term debt of $338 million at September 30, 2011 and $37 million at December 31, 2010.
Schedule of Investment Securities
                                                                 
In Millions
    September 30, 2011     December 31, 2010
            Unrealized     Unrealized                     Unrealized     Unrealized        
    Cost     Gains     Losses     Fair Value     Cost     Gains     Losses     Fair Value  
   
CMS Energy, including consumers                                                
Available for sale
                                                                
SERP
                                                                
Mutual fund
   $ 88     $ —     $ 1     $ 87     $ 62     $ —     $ —     $ 62  
State and municipal bonds
     25       1       —       26       28       —       —       28  
Held to maturity
                                                                
Debt securities
     7       1       —       8       5       1       —       6  
   
Consumers
                                                                
Available for sale
                                                                
SERP
                                                                
Mutual fund
   $ 57     $ —     $ —     $ 57     $ 39     $ —     $ —     $ 39  
State and municipal bonds
     17       —       —       17       17       —       —       17  
CMS Energy common stock
     7       24       —       31       8       26       —       34  
 
Schedule of Proceeds and Gross Realized Gains/Losses from Sales of Available-for-Sale Securities
In Millions  
    Three months ended     Nine months ended  
September 30   2011     2010     2011     2010  
 
CMS Energy, including Consumers
                                
Proceeds from sales of investment securities1
   $ 1     $ —     $ 2     $ 1  
 
Consumers
                                
Proceeds from sales of investment securities1
   $ 1     $ —     $ 1     $ —  
 
1   All of the proceeds related to sales of investments that were held within the SERP and classified as available for sale. Realized losses on these sales were less than $1 million for both CMS Energy and Consumers during each period.
Schedule of Contractual Maturities of Investments
                 
In Millions
    CMS Energy,        
    including Consumers     Consumers  
 
Due one year or less
   $ 2     $ 1  
Due after one year through five years
     9       6  
Due after five years through ten years
     12       8  
Due after ten years
     3       2  
 
Total
   $ 26     $ 17