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Retirement Benefits (Narrative) (Details)
$ in Millions
1 Months Ended 12 Months Ended
Jul. 31, 2018
USD ($)
Dec. 31, 2018
USD ($)
year
Dec. 31, 2017
USD ($)
Dec. 31, 2016
USD ($)
Defined Benefit Plan Disclosure [Line Items]        
Union employees percentage   35.00%    
Consumers Energy Company        
Defined Benefit Plan Disclosure [Line Items]        
Union employees percentage   37.00%    
DB Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Plan amendments   $ 0 $ 0  
Period for gains or losses to be included in market related value   5 years    
ABO   $ 2,200 2,400  
DB Pension Plans | Consumers Energy Company        
Defined Benefit Plan Disclosure [Line Items]        
Period for gains or losses to be included in market related value   5 years    
DB Pension Plans | Equity securities        
Defined Benefit Plan Disclosure [Line Items]        
Target allocation percentage   53.00%    
DB Pension Plans | Fixed-income securities        
Defined Benefit Plan Disclosure [Line Items]        
Target allocation percentage   41.00%    
DB Pension Plans | Multi-asset investments        
Defined Benefit Plan Disclosure [Line Items]        
Target allocation percentage   6.00%    
OPEB Plan        
Defined Benefit Plan Disclosure [Line Items]        
Retirement age requirement | year   55    
Retirement years of service   10 years    
Retirement years of service with disability   15 years    
Ultimate health care cost trend rate   4.75%    
Year health care cost trend rate reaches ultimate trend rate   2027    
Plan amendments $ 26   $ (309)  
Estimated time of amortization of gains losses   10 years 11 years 11 years
Estimated time of prior service cost   9 years    
Service cost discount rate   3.93% 4.89% 4.75%
OPEB Plan | Consumers Energy Company        
Defined Benefit Plan Disclosure [Line Items]        
Retirement age requirement | year   55    
Retirement years of service   10 years    
Retirement years of service with disability   15 years    
Ultimate health care cost trend rate   4.75%    
Year health care cost trend rate reaches ultimate trend rate   2027    
Plan amendments   $ 25 $ (303)  
Pension And OPEB        
Defined Benefit Plan Disclosure [Line Items]        
Amortized net gains and losses in excess of PBO or MRV   10.00%    
Postretirement Health Trusts | Equity securities        
Defined Benefit Plan Disclosure [Line Items]        
Target allocation percentage   50.00%    
Postretirement Health Trusts | Fixed-income securities        
Defined Benefit Plan Disclosure [Line Items]        
Target allocation percentage   30.00%    
Postretirement Health Trusts | Multi-asset investments        
Defined Benefit Plan Disclosure [Line Items]        
Target allocation percentage   20.00%    
Postretirement Life Trusts | Equity securities        
Defined Benefit Plan Disclosure [Line Items]        
Target allocation percentage   42.00%    
Postretirement Life Trusts | Fixed-income securities        
Defined Benefit Plan Disclosure [Line Items]        
Target allocation percentage   28.00%    
Postretirement Life Trusts | Multi-asset investments        
Defined Benefit Plan Disclosure [Line Items]        
Target allocation percentage   30.00%    
Defined Company Contribution Plan        
Defined Benefit Plan Disclosure [Line Items]        
Plan cost, defined contribution plan   $ 26 23 $ 20
Defined Company Contribution Plan | Consumers Energy Company        
Defined Benefit Plan Disclosure [Line Items]        
Plan cost, defined contribution plan   25 22 19
DC SERP        
Defined Benefit Plan Disclosure [Line Items]        
Plan cost, defined contribution plan   $ 1 1  
Minimum years of participation before vesting   5 years    
Trust assets   $ 5 5  
401 (K) Plan        
Defined Benefit Plan Disclosure [Line Items]        
Employer match of eligible wages   3.00%    
Plan cost, defined contribution plan   $ 27 26 24
Employer match of eligible contributions   100.00%    
Secondary employer match of eligible contributions   50.00%    
Secondary employer match of eligible wages   2.00%    
401 (K) Plan | Consumers Energy Company        
Defined Benefit Plan Disclosure [Line Items]        
Plan cost, defined contribution plan   $ 26 $ 25 $ 23
Pension Plan A | DB Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Estimated time of amortization of gains losses   9 years 9 years  
Service cost discount rate   3.85%    
ABO   $ 1,091 $ 1,164  
Pension Plan B | DB Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Estimated time of amortization of gains losses       10 years
Estimated time of amortization of gains (losses) life expectancy   20 years 20 years  
Service cost discount rate     4.53% 4.79%
Minimum | Defined Company Contribution Plan        
Defined Benefit Plan Disclosure [Line Items]        
Employer match of eligible wages   5.00%    
Minimum | DC SERP        
Defined Benefit Plan Disclosure [Line Items]        
Plan contribution percentage   5.00%    
Maximum | Defined Company Contribution Plan        
Defined Benefit Plan Disclosure [Line Items]        
Employer match of eligible wages   7.00%    
Maximum | DC SERP        
Defined Benefit Plan Disclosure [Line Items]        
Plan cost, defined contribution plan       $ 1
Plan contribution percentage   15.00%    
Under Age 65 | OPEB Plan        
Defined Benefit Plan Disclosure [Line Items]        
Health care cost trend rate assumed next fiscal year   7.00% 7.50%  
Under Age 65 | OPEB Plan | Consumers Energy Company        
Defined Benefit Plan Disclosure [Line Items]        
Health care cost trend rate assumed next fiscal year   7.00% 7.50%  
Over Age 65 | OPEB Plan        
Defined Benefit Plan Disclosure [Line Items]        
Health care cost trend rate assumed next fiscal year   7.75% 8.00%  
Over Age 65 | OPEB Plan | Consumers Energy Company        
Defined Benefit Plan Disclosure [Line Items]        
Health care cost trend rate assumed next fiscal year   7.75% 8.00%