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