Retirement Benefits (Schedule of Net Periodic Benefit Cost Not yet Recognized) (Details) (USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Pension And DB SERP [Member]
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net (Gains) Losses, before Tax	$ 1,095	$ 1,014
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net Prior Service Cost (Credit), before Tax	13	17
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net (Gains) Losses, before Tax	98	81
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax		2
Total amounts not yet recognized as component of net periodic benefit cost	1,206	1,114
Pension And DB SERP [Member] | Consumers Energy Company [Member]
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net (Gains) Losses, before Tax	1,095	1,014
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net Prior Service Cost (Credit), before Tax	13	17
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax	38	27
Total amounts not yet recognized as component of net periodic benefit cost	1,146	1,058
Other Postretirement Benefit Plans, Defined Benefit [Member]
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net (Gains) Losses, before Tax	704	766
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net Prior Service Cost (Credit), before Tax	(112)	(132)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net (Gains) Losses, before Tax	(7)	(5)
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), Net Prior Service Cost (Credit), before Tax	(3)	(3)
Total amounts not yet recognized as component of net periodic benefit cost	582	626
Other Postretirement Benefit Plans, Defined Benefit [Member] | Consumers Energy Company [Member]
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net (Gains) Losses, before Tax	704	766
Pension and Other Postretirement Benefit Plans, Regulatory Assets, Net Prior Service Cost (Credit), before Tax	(112)	(132)
Total amounts not yet recognized as component of net periodic benefit cost	$ 592	$ 634