Retirement Benefits (Narrative) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended				9 Months Ended	3 Months Ended		9 Months Ended				9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member]	Dec. 31, 2012 Other Postretirement Benefit Plans, Defined Benefit [Member]	Sep. 30, 2013 Other Postretirement Benefit Plans, Defined Benefit [Member] Remeasurement As Of July 1, 2013 [Member]	Sep. 30, 2013 Consumers Energy Company [Member]	Sep. 30, 2012 Consumers Energy Company [Member]	Sep. 30, 2013 Consumers Energy Company [Member]	Sep. 30, 2012 Consumers Energy Company [Member]	Sep. 30, 2013 Consumers Energy Company [Member] Other Postretirement Benefit Plans, Defined Benefit [Member]	Dec. 31, 2012 Consumers Energy Company [Member] Other Postretirement Benefit Plans, Defined Benefit [Member]	Sep. 30, 2013 Consumers Energy Company [Member] Other Postretirement Benefit Plans, Defined Benefit [Member] Remeasurement As Of July 1, 2013 [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate					5.10%	4.40%						5.10%	4.40%
Deacrease in liability							$ 638							$ 614
Regulatory asset decrease							580							580
Increase in regulatory liabilities							34							34
Decrease in AOCL							24
Decrease in expected cost							48							46
Decrease deferred tax assets							148							144
Reduced regulatory income tax liabilities							144							144
Increased income tax expense	$ 91	$ 98	$ 240	$ 203			$ 4	$ 102	$ 107	$ 272	$ 239