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Retirement Benefits (Schedule Of Net Benefit Costs) (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Pension And DB SERP [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 42 $ 50 $ 42
Interest cost 90 108 105
Expected return on plan assets (147) (138) (135)
Amortization of Net loss 71 97 60
Amortization of Prior service cost (credit) 4 1 1
Net periodic cost (credit) 60 118 73
Pension And DB SERP [Member] | Consumers Energy Company [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 41 49 41
Interest cost 87 103 100
Expected return on plan assets (143) (134) (131)
Amortization of Net loss 68 93 59
Amortization of Prior service cost (credit) 4 1 1
Net periodic cost (credit) 57 112 70
DB Pension Plan [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 42 49  
Interest cost 85 102  
OPEB [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 18 25 20
Interest cost 46 58 56
Expected return on plan assets (85) (91) (88)
Amortization of Net loss 21 21 2
Amortization of Prior service cost (credit) (41) (41) (41)
Net periodic cost (credit) (41) (28) (51)
OPEB [Member] | Consumers Energy Company [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 17 25 20
Interest cost 45 56 54
Expected return on plan assets (80) (86) (83)
Amortization of Net loss 22 22 3
Amortization of Prior service cost (credit) (40) (40) (40)
Net periodic cost (credit) $ (36) $ (23) $ (46)