Derivative Instruments (Assets and Liabilities Fair Value of Derivative Instruments) (Details) (USD $)
In Millions
Sep. 30, 2011
Dec. 31, 2010
Impact of master netting arrangements less than$ 1$ 1
Impact of cash margin deposits on derivative liabilities less than11
Consumers Energy Company [Member] | Other Assets [Member] | Commodity Contracts [Member]
  
Derivative Assets21
Other Assets [Member] | Commodity Contracts [Member]
  
Derivative Assets2[1]1[1]
Consumers Energy Company [Member] | Other Liabilities [Member] | Commodity Contracts [Member]
  
Derivative Liabilities  
Other Liabilities [Member] | Commodity Contracts [Member]
  
Derivative Liabilities$ 3[2]$ 4[2]
[1]1 Assets and liabilities are presented gross and exclude the impact of offsetting derivative assets and liabilities under master netting agreements, which was less than $1 million at September 30, 2011 and December 31, 2010.
[2]2 Liabilities exclude the impact of offsetting cash margin deposits paid by CMS ERM to other parties, which was less than $1 million at September 30, 2011 and December 31, 2010. CMS Energy presents these liabilities net of these impacts on its consolidated balance sheets.