Fair Value Measurements (Assets and Liabilties Measured at Fair Value on a Recurring Basis) (Details) (USD $)
In Millions, unless otherwise specified
Sep. 30, 2011
Dec. 31, 2010
Cash equivalents$ 512$ 183
Restricted cash equivalents146
Nonqualified deferred compensation plan assets46
Available for sale - fair value11390
Commodity contracts2[1]1[2]
Asset Total646[3]287[4]
Nonqualified deferred compensation plan liabilities46
Commodity contracts3[5]4[6]
Liabilities Total7[7]10[8]
Assets classified as level three as a percentage of total assets measured at fair value less than (except Consumers 12/31/10)1.00%1.00%
Impact of master netting arrangements less than11
Impact of offsetting derivative assets and liabilities under master netting arrangements and offsetting cash margin deposits paid by CMS ERM less than11
Liabilities classified as level three as a percentage of total liabilities measured at fair value43.00%40.00%
Consumers Energy Company [Member] | CMS Energy Common Stock [Member]
  
Available for sale - fair value3134
Consumers Energy Company [Member] | CMS Energy Common Stock [Member] | Fair Value, Inputs, Level 1 [Member]
  
Available for sale - fair value3134
Consumers Energy Company [Member] | CMS Energy Common Stock [Member] | Fair Value, Inputs, Level 2 [Member]
  
Available for sale - fair value  
Consumers Energy Company [Member] | CMS Energy Common Stock [Member] | Fair Value, Inputs, Level 3 [Member]
  
Available for sale - fair value  
Consumers Energy Company [Member] | Fair Value, Inputs, Level 1 [Member] | Supplemental Executive Retirement Plan [Member] | Cash Equivalents [Member]
  
Available for sale - fair value11
Consumers Energy Company [Member] | Fair Value, Inputs, Level 1 [Member] | Supplemental Executive Retirement Plan [Member] | Mutual Fund [Member]
  
Available for sale - fair value5739
Consumers Energy Company [Member] | Fair Value, Inputs, Level 1 [Member] | Supplemental Executive Retirement Plan [Member] | State and Municipal Bonds [Member]
  
Available for sale - fair value  
Consumers Energy Company [Member] | Fair Value, Inputs, Level 2 [Member] | Supplemental Executive Retirement Plan [Member] | Cash Equivalents [Member]
  
Available for sale - fair value  
Consumers Energy Company [Member] | Fair Value, Inputs, Level 2 [Member] | Supplemental Executive Retirement Plan [Member] | Mutual Fund [Member]
  
Available for sale - fair value  
Consumers Energy Company [Member] | Fair Value, Inputs, Level 2 [Member] | Supplemental Executive Retirement Plan [Member] | State and Municipal Bonds [Member]
  
Available for sale - fair value1717
Consumers Energy Company [Member] | Fair Value, Inputs, Level 3 [Member] | Supplemental Executive Retirement Plan [Member] | Cash Equivalents [Member]
  
Available for sale - fair value  
Consumers Energy Company [Member] | Fair Value, Inputs, Level 3 [Member] | Supplemental Executive Retirement Plan [Member] | Mutual Fund [Member]
  
Available for sale - fair value  
Consumers Energy Company [Member] | Fair Value, Inputs, Level 3 [Member] | Supplemental Executive Retirement Plan [Member] | State and Municipal Bonds [Member]
  
Available for sale - fair value  
Consumers Energy Company [Member] | Supplemental Executive Retirement Plan [Member]
  
Available for sale - fair value17 
Consumers Energy Company [Member] | Supplemental Executive Retirement Plan [Member] | Cash Equivalents [Member]
  
Available for sale - fair value11
Consumers Energy Company [Member] | Supplemental Executive Retirement Plan [Member] | Mutual Fund [Member]
  
Available for sale - fair value5739
Consumers Energy Company [Member] | Supplemental Executive Retirement Plan [Member] | State and Municipal Bonds [Member]
  
Available for sale - fair value1717
Fair Value, Inputs, Level 1 [Member] | Supplemental Executive Retirement Plan [Member] | Cash Equivalents [Member]
  
Available for sale - fair value11
Fair Value, Inputs, Level 1 [Member] | Supplemental Executive Retirement Plan [Member] | Mutual Fund [Member]
  
Available for sale - fair value8762
Fair Value, Inputs, Level 1 [Member] | Supplemental Executive Retirement Plan [Member] | State and Municipal Bonds [Member]
  
Available for sale - fair value  
Fair Value, Inputs, Level 2 [Member] | Supplemental Executive Retirement Plan [Member] | Cash Equivalents [Member]
  
Available for sale - fair value  
Fair Value, Inputs, Level 2 [Member] | Supplemental Executive Retirement Plan [Member] | Mutual Fund [Member]
  
Available for sale - fair value  
Fair Value, Inputs, Level 2 [Member] | Supplemental Executive Retirement Plan [Member] | State and Municipal Bonds [Member]
  
Available for sale - fair value2628
Fair Value, Inputs, Level 3 [Member] | Supplemental Executive Retirement Plan [Member] | Cash Equivalents [Member]
  
Available for sale - fair value  
Fair Value, Inputs, Level 3 [Member] | Supplemental Executive Retirement Plan [Member] | Mutual Fund [Member]
  
Available for sale - fair value  
Fair Value, Inputs, Level 3 [Member] | Supplemental Executive Retirement Plan [Member] | State and Municipal Bonds [Member]
  
Available for sale - fair value  
Supplemental Executive Retirement Plan [Member]
  
Available for sale - fair value26 
Supplemental Executive Retirement Plan [Member] | Cash Equivalents [Member]
  
Available for sale - fair value11
Supplemental Executive Retirement Plan [Member] | Mutual Fund [Member]
  
Available for sale - fair value8762
Supplemental Executive Retirement Plan [Member] | State and Municipal Bonds [Member]
  
Available for sale - fair value2628
Consumers Energy Company [Member] | Fair Value, Inputs, Level 1 [Member]
  
Cash equivalents28219
Restricted cash equivalents136
Nonqualified deferred compensation plan assets34
Commodity contracts  
Asset Total387[9]103[10]
Nonqualified deferred compensation plan liabilities34
Liabilities Total34
Fair Value, Inputs, Level 1 [Member]
  
Cash equivalents512183
Restricted cash equivalents146
Nonqualified deferred compensation plan assets46
Commodity contracts [1] [2]
Asset Total618[3]258[4]
Nonqualified deferred compensation plan liabilities46
Commodity contracts [5] [6]
Liabilities Total4[7]6[8]
Consumers Energy Company [Member] | Fair Value, Inputs, Level 2 [Member]
  
Cash equivalents  
Restricted cash equivalents  
Nonqualified deferred compensation plan assets  
Commodity contracts  
Asset Total17[9]17[10]
Nonqualified deferred compensation plan liabilities  
Liabilities Total  
Fair Value, Inputs, Level 2 [Member]
  
Cash equivalents  
Restricted cash equivalents  
Nonqualified deferred compensation plan assets  
Commodity contracts [1] [2]
Asset Total26[3]28[4]
Nonqualified deferred compensation plan liabilities  
Commodity contracts [5] [6]
Liabilities Total [7] [8]
Consumers Energy Company [Member] | Fair Value, Inputs, Level 3 [Member]
  
Cash equivalents  
Restricted cash equivalents  
Nonqualified deferred compensation plan assets  
Commodity contracts21
Asset Total2[9]1[10]
Nonqualified deferred compensation plan liabilities  
Liabilities Total  
Fair Value, Inputs, Level 3 [Member]
  
Cash equivalents  
Restricted cash equivalents  
Nonqualified deferred compensation plan assets  
Commodity contracts2[1]1[2]
Asset Total2[3]1[4]
Nonqualified deferred compensation plan liabilities  
Commodity contracts3[5]4[6]
Liabilities Total3[7]4[8]
Consumers Energy Company [Member]
  
Cash equivalents28219
Restricted cash equivalents136
Nonqualified deferred compensation plan assets34
Available for sale - fair value10590
Commodity contracts21
Asset Total406[9]121[10]
Nonqualified deferred compensation plan liabilities34
Liabilities Total$ 3$ 4
Assets classified as level three as a percentage of total assets measured at fair value less than (except Consumers 12/31/10)1.00%1.00%
[1]This amount is gross and excludes the impact of offsetting derivative assets and liabilities under master netting arrangements, which was less than $1million at September 30, 2011.
[2]This amount is gross and excludes the impact of offsetting derivative assets and liabilities under master netting arrangements, which was less than $1million at December 31, 2010.
[3]At September 30, 2011, CMS Energy's assets classified as Level 3 represented less than one percent of CMS Energy's total assets measured at fair value.
[4]At December 31, 2010, CMS Energy's assets classified as Level 3 represented less than one percent of CMS Energy's total assets measured at fair value.
[5]This amount is gross and excludes the impact of offsetting derivative assets and liabilities under master netting arrangements and offsetting cash margin deposits paid by CMS ERM to other parties, which was less than $1million at September 30, 2011.
[6]This amount is gross and excludes the impact of offsetting derivative assets and liabilities under master netting arrangements and offsetting cash margin deposits paid by CMS ERM to other parties, which was less than $1million at December 31, 2010.
[7]At September 30, 2011, CMS Energy's liabilities classified as Level 3 represented 43 percent of CMS Energy's total liabilities measured at fair value. The Level 3 liabilities consisted primarily of an electricity sales agreement held by CMS ERM.
[8]At December 31, 2010, CMS Energy's liabilities classified as Level 3 represented 40 percent of CMS Energy's total liabilities measured at fair value. The Level 3 liabilities consisted primarily of an electricity sales agreement held by CMS ERM.
[9]At September 30, 2011, Consumers' assets classified as Level 3 represented less than one percent of Consumers' total assets measured at fair value.
[10]At December 31, 2010, Consumers' assets classified as Level 3 represented one percent of Consumers' total assets measured at fair value.