Retirement Benefits (Schedule of Plan Contributions) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012		Dec. 31, 2011		Dec. 31, 2010
Postretirement benefits contributions	$ 72		$ 323		$ 463
Consumers Energy Company [Member]
Postretirement benefits contributions	68		315		447
Other Postretirement Benefit Plans, Defined Benefit [Member]
Postretirement benefits contributions	65	[1]	67	[1]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	74
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Postretirement benefits contributions	64	[1]	66	[1]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	73
Pension Plans, Defined Benefit [Member]
Postretirement benefits contributions			250	[2]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	50
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Postretirement benefits contributions			245	[2]
Defined Benefit Plan, Estimated Future Employer Contributions in Next Fiscal Year	49
VEBA Trust [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Postretirement benefits contributions	45	[1]	48	[1]
VEBA Trust [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Postretirement benefits contributions	45	[1]	47	[1]
401 (h) Component [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Postretirement benefits contributions	20	[1]	19	[1]
401 (h) Component [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Postretirement benefits contributions	$ 19	[1]	$ 19	[1]
Equity Securities [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Target Plan Asset Allocations	50.00%
Equity Securities [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Target Plan Asset Allocations	50.00%
Equity Securities [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Target Plan Asset Allocations	50.00%		50.00%
Equity Securities [Member] | Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Target Plan Asset Allocations	50.00%		50.00%
Fixed Income Funds [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Target Plan Asset Allocations	20.00%
Fixed Income Funds [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Target Plan Asset Allocations	20.00%
Fixed Income Funds [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Target Plan Asset Allocations	30.00%		30.00%
Fixed Income Funds [Member] | Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Target Plan Asset Allocations	30.00%		30.00%
Alternative Strategy Investments [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Target Plan Asset Allocations	30.00%
Alternative Strategy Investments [Member] | Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Target Plan Asset Allocations	30.00%
Alternative Strategy Investments [Member] | Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Target Plan Asset Allocations	20.00%		20.00%
Alternative Strategy Investments [Member] | Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Target Plan Asset Allocations	20.00%		20.00%

[1] CMS Energy, including Consumers, plans to contribute $74 million to the OPEB plan in 2013, of which Consumers plans to contribute $73 million.

[2] CMS Energy, including Consumers, planned to contribute $50 million to the Pension Plan in 2013, of which Consumers planned to contribute $49 million. This Pension Plan contribution was made in January 2013.