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Revenue (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Uncollectable expense $ 6 $ 7  
Unbilled receivables 341   $ 409
Consumers Energy Company      
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]      
Uncollectable expense 6 $ 7  
Unbilled receivables $ 341   $ 409