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Notes Receivable (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
CMS Energy Note Payable      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Interest rate     4.10%
EnerBank      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Unearned income $ 122 $ 128  
Sale of notes receivable 279    
Gain on sale of notes receivable 10    
Delinquent loans 24 32  
EnerBank | Unfunded Loan Commitment      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Unfunded loan commitments 365    
Allowance for expected credit loss on off balance sheet commitments $ 7    
EnerBank | Credit Concentration Risk | Loans Originated Within Last Five Years      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Concentration risk, percentage 96.00%    
EnerBank | Credit Concentration Risk | FICO Score, Between Good And Excellent      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Concentration risk, percentage 86.00%    
EnerBank | Retail Installment Contracts      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Consumer retail installment contracts $ 33    
EnerBank | Interest Rate Swap, Notes Receivable | Fair Value Hedging | Designated as Hedging Instrument      
Accounts, Notes, Loans and Financing Receivable [Line Items]      
Notional amount $ 134 $ 134