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Notes Receivable (Narrative) (Details) - USD ($)
$ in Millions
3 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Accounts, Notes, Loans and Financing Receivable [Line Items]    
EnerBank notes receivable, net of allowance for loan losses $ 241 $ 223
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]    
EnerBank notes receivable, net of allowance for loan losses $ 241 242
Delinquent loans 31 33
EnerBank | Retail Installment Contracts    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Financing receivable purchases 9  
EnerBank | Receivables Excluding Held for Sale    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Unearned income 130 134
EnerBank notes receivable, net of allowance for loan losses 241 223
EnerBank | Receivables Held for Sale    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
EnerBank notes receivable, net of allowance for loan losses 0 $ 19
Fair Value Hedging | Designated as Hedging Instrument | EnerBank | Interest Rate Swap Notes Receivable    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Interest rate swap $ 134  
Credit Concentration Risk | FICO Score, between Good and Excellent | EnerBank    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Concentration risk percentage 85.00%  
Unfunded Loan Commitment    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Off-balance sheet unfunded loan commitments $ 182  
Allowance for expected credit loss on off balance sheet commitments $ 3  
Loans Originated Within Last Five Years | Credit Concentration Risk | EnerBank    
Accounts, Notes, Loans and Financing Receivable [Line Items]    
Concentration risk percentage 97.00%