Retirement Benefits (Schedule of Expected Benefit Payments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	$ 120
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	128
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	138
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	144
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	149
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	793
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	117
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	125
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	134
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	140
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	146
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	773
Supplemental Executive Retirement Plan [Member]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	7
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	8
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	8
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	8
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	8
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	46
Supplemental Executive Retirement Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	4
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	4
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	4
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	4
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	4
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	20
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	61	[1]
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	65	[1]
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	70	[1]
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	74	[1]
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	79	[1]
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	451	[1]
Prescription Drug Subsidy Receipts, Next Twelve Months	6
Prescription Drug Subsidy Receipts, Year Two	7
Prescription Drug Subsidy Receipts, Year Three	7
Prescription Drug Subsidy Receipts, Year Four	8
Prescription Drug Subsidy Receipts, Year Five	8
Prescription Drug Subsidy Receipts, after Year Five	51
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Expected Future Benefit Payments, Next Twelve Months	59	[1]
Defined Benefit Plan, Expected Future Benefit Payments, Year Two	63	[1]
Defined Benefit Plan, Expected Future Benefit Payments, Year Three	67	[1]
Defined Benefit Plan, Expected Future Benefit Payments, Year Four	71	[1]
Defined Benefit Plan, Expected Future Benefit Payments, Year Five	76	[1]
Defined Benefit Plan, Expected Future Benefit Payments, Five Fiscal Years Thereafter	432	[1]
Prescription Drug Subsidy Receipts, Next Twelve Months	6
Prescription Drug Subsidy Receipts, Year Two	6
Prescription Drug Subsidy Receipts, Year Three	7
Prescription Drug Subsidy Receipts, Year Four	7
Prescription Drug Subsidy Receipts, Year Five	8
Prescription Drug Subsidy Receipts, after Year Five	$ 48

[1] CMS Energy’s and Consumers’ OPEB benefit payments are net of employee contributions and expected Medicare Part D prescription drug subsidy payments. For CMS Energy, subsidies to be received are estimated to be $6 million for 2013, $7 million for each of 2014 and 2015, $8 million for each of 2016 and 2017, and $51 million combined for 2018 through 2022. For Consumers, subsidies to be received are estimated to be $6 million for each of 2013 and 2014, $7 million for each of 2015 and 2016, $8 million for 2017, and $48 million combined for 2018 through 2022.