Retirement Benefits (Schedule of Expected Benefit Payments) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2011
Pension Plans, Defined Benefit [Member]
Defined Benefit Plan, Expected Future Benefit Payments in Year One	$ 111
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	120
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	129
Defined Benefit Plan, Expected Future Benefit Payments in Year Four	138
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	144
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	777
Pension Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Expected Future Benefit Payments in Year One	108
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	117
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	126
Defined Benefit Plan, Expected Future Benefit Payments in Year Four	134
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	140
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	757
Supplemental Executive Retirement Plan [Member]
Defined Benefit Plan, Expected Future Benefit Payments in Year One	7
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	7
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	7
Defined Benefit Plan, Expected Future Benefit Payments in Year Four	8
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	8
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	44
Supplemental Executive Retirement Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Expected Future Benefit Payments in Year One	4
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	4
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	4
Defined Benefit Plan, Expected Future Benefit Payments in Year Four	4
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	4
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	23
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan, Expected Future Benefit Payments in Year One	70	[1]
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	73	[1]
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	76	[1]
Defined Benefit Plan, Expected Future Benefit Payments in Year Four	80	[1]
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	83	[1]
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	466	[1]
Prescription Drug Subsidy Receipts, Year One	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, Five Fiscal Years Thereafter	49
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Defined Benefit Plan, Expected Future Benefit Payments in Year One	67	[1]
Defined Benefit Plan, Expected Future Benefit Payments in Year Two	70	[1]
Defined Benefit Plan, Expected Future Benefit Payments in Year Three	73	[1]
Defined Benefit Plan, Expected Future Benefit Payments in Year Four	76	[1]
Defined Benefit Plan, Expected Future Benefit Payments in Year Five	80	[1]
Defined Benefit Plan, Expected Future Benefit Payments in Five Fiscal Years Thereafter	445	[1]
Prescription Drug Subsidy Receipts, Year One	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, Five Fiscal Years Thereafter	$ 47

[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 2012, $7 million for each of 2013 and 2014, $8 million for each of 2015 and 2016, and $49 million combined for 2017 through 2021. For Consumers, subsidies to be received are estimated to be $6 million for each of 2012 and 2013, $7 million for each of 2014 and 2015, $8 million for 2016, and $47 million combined for 2017 through 2021.