Notes Receivable (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts, Notes, Loans and Financing Receivable [Line Items]
Sale of notes receivable	$ 67,000,000	$ 0	$ 50,000,000
Notes and accrued interest receivable	223,000,000	$ 233,000,000
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]
Sale of notes receivable	67,000,000
Notes and accrued interest receivable	242,000,000	$ 233,000,000
Unearned income	134,000,000	102,000,000
Delinquent loans	33,000,000	21,000,000
EnerBank | EnerBank notes receivable held for sale
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notes and accrued interest receivable	19,000,000	$ 0
Unearned income	2,000,000
EnerBank | Retail Installment Contracts
Accounts, Notes, Loans and Financing Receivable [Line Items]
Consumer retail installment contracts	373,000,000
EnerBank | Interest Rate Swap, Notes Receivable | Fair Value Hedging | Designated as Hedging Instrument
Accounts, Notes, Loans and Financing Receivable [Line Items]
Notional amount	$ 134,000,000