Revenue (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Bad debt expense	$ 90	$ 67	$ 54
Regulatory assets	2,653	2,489
Unbilled receivables	437	426
Regulatory liabilities	151	87
Consumers Energy Company
Disaggregation of Revenue [Line Items]
Bad debt expense			29
Regulatory assets	2,653	2,489
Unbilled receivables	437	426
Regulatory liabilities	151	87
Consumers Energy Company | Reserve for customer refunds
Disaggregation of Revenue [Line Items]
Regulatory liabilities	28	2
Consumers Energy Company | COVID-19 costs accounting deferral
Disaggregation of Revenue [Line Items]
Regulatory assets	4	0
Accounts Receivable
Disaggregation of Revenue [Line Items]
Bad debt expense	33	29	29
Accounts Receivable | Consumers Energy Company
Disaggregation of Revenue [Line Items]
Bad debt expense	$ 33	$ 29	$ 29