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Regulatory Matters (Schedule of the Components of Regulatory Assets and Liabilities) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Regulatory assets, current $ 35 $ 1
Regulatory assets, noncurrent 2,287 2,466
Regulatory liabilities, current 25 125
Regulatory liabilities, noncurrent 2,101 1,875
Consumers Energy Company [Member]
   
Regulatory assets, current 35 1
Regulatory assets, noncurrent 2,287 2,466
Total regulatory asset 2,322 2,467
Regulatory liabilities, current 25 125
Regulatory liabilities, noncurrent 2,101 1,875
Total regulatory liabilities 2,126 2,000
Requested Energy Optimization Incentive for Exceeding Savings Targets 17  
DOE Settlement [Member] | Consumers Energy Company [Member]
   
Current Regulatory Liability Recovery End Date 2013  
Regulatory liabilities, current 23 120
Cost Of Removal [Member] | Consumers Energy Company [Member]
   
Regulatory liabilities, noncurrent 1,441 1,364
Income Taxes, Net [Member] | Consumers Energy Company [Member]
   
Regulatory liabilities, noncurrent 336 181
Renewable Energy Plan [Member] | Consumers Energy Company [Member]
   
Noncurrent Regulatory Liability, Recovery End Date 2028  
Regulatory liabilities, noncurrent 175 161
ARO [Member] | Consumers Energy Company [Member]
   
Regulatory liabilities, noncurrent 103 113
Energy Optimization Plan [Member] | Consumers Energy Company [Member]
   
Noncurrent Regulatory Liability, Recovery End Date 2014  
Regulatory liabilities, noncurrent 34 45
Other Regulatory Liabilities [Member] | Consumers Energy Company [Member]
   
Current Regulatory Liability Recovery End Date 2013  
Regulatory liabilities, current 2 5
Regulatory liabilities, noncurrent 12 11
Postretirement Benefits [Member] | Consumers Energy Company [Member]
   
Regulatory assets, noncurrent 1,700 [1] 1,665 [1]
Securitized Costs [Member] | Consumers Energy Company [Member]
   
Noncurrent Regulatory Asset Recovery End Date 2016 [2]  
Regulatory assets, noncurrent 192 [2] 252 [2]
MGP Sites [Member] | Consumers Energy Company [Member]
   
Regulatory assets, noncurrent 152 [2] 156 [2]
ARO [Member] | Consumers Energy Company [Member]
   
Regulatory assets, noncurrent 123 [3] 114 [3]
Unamortized Debt Costs [Member] | Consumers Energy Company [Member]
   
Regulatory assets, noncurrent 55 [2] 44 [2]
Gas Revenue Decoupling Mechanism 2011-2012 [Member] | Consumers Energy Company [Member]
   
Noncurrent Regulatory Asset Recovery End Date 2014  
Regulatory assets, noncurrent 17 21
Energy Optimization Plan Incentive [Member] | Consumers Energy Company [Member]
   
Regulatory Current Asset, End Date for Recovery 2013  
Noncurrent Regulatory Asset Recovery End Date 2014  
Regulatory assets, current 15  
Regulatory assets, noncurrent 17 26
Gas Storage Inventory Adjustments [Member] | Consumers Energy Company [Member]
   
Regulatory assets, noncurrent 15 [2] 14 [2]
2010-2011 Gas Revenue Decoupling Mechanism [Member] | Consumers Energy Company [Member]
   
Regulatory Current Asset, End Date for Recovery 2013  
Regulatory assets, current 16  
Cancelled Coal-Fueled Plant Costs [Member] | Consumers Energy Company [Member]
   
Regulatory Current Asset, End Date for Recovery 2013 [4]  
Noncurrent Regulatory Asset Recovery End Date 2015 [4]  
Regulatory assets, current 4 [4]  
Regulatory assets, noncurrent 7 [4]  
Major Maintenance [Member] | Consumers Energy Company [Member]
   
Regulatory assets, noncurrent 5 [4]  
Big Rock Decommissioning [Member] | Consumers Energy Company [Member]
   
Noncurrent Regulatory Asset Recovery End Date 2012 [2]  
Regulatory assets, noncurrent   85 [2]
Electric Revenue Decoupling Mechanism [Member] | Consumers Energy Company [Member]
   
Regulatory assets, noncurrent   59
Stranded Costs [Member] | Consumers Energy Company [Member]
   
Noncurrent Regulatory Asset Recovery End Date 2012 [4]  
Regulatory assets, noncurrent   23 [4]
Other Regulatory Assets [Member] | Consumers Energy Company [Member]
   
Regulatory Current Asset, End Date for Recovery 2012 [4]  
Regulatory assets, current   1 [4]
Regulatory assets, noncurrent $ 4 [4] $ 7 [4]
[1] This regulatory asset is offset partially by liabilities. The net amount is included in rate base, thereby providing a return.
[2] These regulatory assets represent expenditures for which the MPSC has provided, or Consumers expects, recovery without a return on investment.
[3] Consumers records a regulatory asset and a regulatory liability for timing differences between the recognition of AROs for financial reporting purposes and the recovery of these costs from customers. Although the regulatory asset for AROs does not provide a return on investment, the underlying asset retirement costs are capitalized as part of the carrying amount of the related asset and included in rate base, thereby providing a return.
[4] These regulatory assets either are included in rate base (or are expected to be included, for costs incurred subsequent to the most recently approved rate case), thereby providing a return on expenditures, or provide a specific return on investment authorized by the MPSC.