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Income Taxes (Schedule Of Effective Income Rate Reconciliation) (Details) - USD ($)
$ in Millions
9 Months Ended 12 Months Ended
Sep. 30, 2021
Sep. 30, 2020
Dec. 31, 2020
Income Taxes [Line Items]      
U.S. federal income tax rate 21.00% 21.00%  
Increase (decrease) in income taxes from:      
State and local income taxes, net of federal effect 5.50% 4.80%  
TCJA excess deferred taxes (5.80%) (4.30%)  
Production tax credits (4.80%) (3.30%)  
Accelerated flow-through of regulatory tax benefits (3.20%) (1.60%)  
Research and development tax credits, net (0.30%) (1.60%)  
Refund of alternative minimum tax sequestration 0.00% (1.40%)  
Other, net 0.40% 0.10%  
Effective tax rate 12.80% 13.70%  
Refund of alternative minimum tax sequestration     $ (9)
Consumers Energy Company      
Income Taxes [Line Items]      
U.S. federal income tax rate 21.00% 21.00%  
Increase (decrease) in income taxes from:      
State and local income taxes, net of federal effect 5.20% 5.00%  
TCJA excess deferred taxes (4.60%) (3.60%)  
Production tax credits (3.50%) (1.90%)  
Accelerated flow-through of regulatory tax benefits (2.20%) (1.10%)  
Research and development tax credits, net (0.30%) (1.30%)  
Other, net (0.20%) (0.30%)  
Effective tax rate 15.40% 17.80%  
Research Tax Credit Carryforward      
Increase (decrease) in income taxes from:      
Increase (decrease) in tax credit carryforward     9
Research Tax Credit Carryforward | Consumers Energy Company      
Increase (decrease) in income taxes from:      
Increase (decrease) in tax credit carryforward     $ 8