Income Taxes (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Tax Benefits [Line Items]
Increase (decrease) in tax credit carryforward	$ (7,000,000)
Tax refunds	69,000,000
Interest and penalties	0	$ 0	$ 0
Consumers Energy Company
Income Tax Benefits [Line Items]
Future tax benefit	9,000,000
Interest and penalties	0	$ 0	$ 0
Local Tax Authority
Income Tax Benefits [Line Items]
Valuation allowance - loss carryforward	$ 1,000,000