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.