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Retirement Benefits (Narrative) (Details)
$ in Millions
12 Months Ended
Dec. 31, 2015
USD ($)
item
Dec. 31, 2014
USD ($)
item
Dec. 31, 2013
USD ($)
item
Defined Benefit Plan Disclosure [Line Items]      
Postretirement benefits contributions $ 262 $ 32 $ 229
Union employees percentage 40.00%    
Consumers Energy Company [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Postretirement benefits contributions $ 243 29 222
Union employees percentage 42.00%    
Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Trust assets $ 2,013 $ 1,979 $ 1,964
Estimated time of amortization of gains losses 10 years 10 years 10 years
Postretirement benefits contributions $ 225    
Pension Plan [Member] | Consumers Energy Company [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Estimated time of amortization of gains losses 10 years 10 years 10 years
Postretirement benefits contributions $ 209    
OPEB [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Trust assets $ 1,208 $ 1,265 $ 1,218
Retirement age requirement | item 55 55 55
Retirement years of service 10 years    
Retirement years of service with disability 15 years    
Ultimate health care cost trend rate 4.75%    
Year that rate reaches ultimate trend rate 2027    
Estimated time of amortization of gains losses 13 years 13 years 13 years
Estimated time of prior service cost 10 years   10 years
Postretirement benefits contributions $ 29 $ 25  
OPEB [Member] | Consumers Energy Company [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Trust assets $ 1,133 $ 1,186 $ 1,141
Retirement age requirement | item 55 55 55
Retirement years of service 10 years    
Retirement years of service with disability 15 years    
Ultimate health care cost trend rate 4.75%    
Year that rate reaches ultimate trend rate 2027    
Estimated time of amortization of gains losses 13 years 13 years 13 years
Estimated time of prior service cost 10 years   10 years
Postretirement benefits contributions $ 29 $ 25  
Pension And OPEB [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Amortized net gains and losses in excess of PBO or MRV 10.00% 10.00% 10.00%
Pension And OPEB [Member] | Consumers Energy Company [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Amortized net gains and losses in excess of PBO or MRV 10.00% 10.00% 10.00%
Defined Company Contribution Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Employer contribution, percentage of base pay 6.00% 6.00% 6.00%
Plan cost, defined contribution plan $ 16 $ 13 $ 10
Defined Company Contribution Plan [Member] | Consumers Energy Company [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Employer contribution, percentage of base pay 6.00% 6.00% 6.00%
Plan cost, defined contribution plan $ 16 $ 13 $ 10
DC SERP [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Minimum years of participation before vesting 5 years    
Trust assets $ 2 2  
DC SERP [Member] | Consumers Energy Company [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Minimum years of participation before vesting 5 years    
Trust assets $ 2 2  
401 (K) Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan cost, defined contribution plan $ 19 $ 18 $ 17
Employer match of eligible contributions 60.00% 60.00% 60.00%
Employer match of eligible wages 6.00% 6.00% 6.00%
401 (K) Plan [Member] | Consumers Energy Company [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan cost, defined contribution plan $ 19 $ 18 $ 17
Employer match of eligible contributions 60.00% 60.00% 60.00%
Employer match of eligible wages 6.00% 6.00% 6.00%
Minimum [Member] | DC SERP [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan contribution percentage 5.00%    
Minimum [Member] | DC SERP [Member] | Consumers Energy Company [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan contribution percentage 5.00%    
Maximum [Member] | DC SERP [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan cost, defined contribution plan $ 1 $ 1 $ 1
Plan contribution percentage 15.00%    
Maximum [Member] | DC SERP [Member] | Consumers Energy Company [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Plan cost, defined contribution plan $ 1 $ 1 $ 1
Plan contribution percentage 15.00%    
Under Age 65 [Member] | OPEB [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Health care cost trend rate assumed for next fiscal year 7.25% 6.50%  
Under Age 65 [Member] | OPEB [Member] | Consumers Energy Company [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Health care cost trend rate assumed for next fiscal year 7.25% 6.50%  
Over Age 65 [Member] | OPEB [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Health care cost trend rate assumed for next fiscal year 8.00% 6.50%  
Over Age 65 [Member] | OPEB [Member] | Consumers Energy Company [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Health care cost trend rate assumed for next fiscal year 8.00% 6.50%