Income Taxes (Narrative) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Benefits [Line Items]
Increase to income tax expense	$ 5,000,000
Increase (decrease) to deferred income tax expense related to tax rate changes		$ 148,000,000
Increase (decrease) to deferred tax liabilities related to tax rate changes		1,300,000,000
Alternative minimum tax, recovery period	4 years
Valuation allowance	$ 8,000,000	15,000,000
Alternative minimum tax credit reclassification	68,000,000
Interest and penalties	0	0	$ 0
General business credits
Income Tax Benefits [Line Items]
Valuation allowance - general business credits	3,000,000
Consumers Energy Company
Income Tax Benefits [Line Items]
Increase to income tax expense	1,000,000
Net regulatory tax liability	3,836,000,000	3,795,000,000
Increase (decrease) to deferred income tax expense related to tax rate changes		33,000,000
Interest and penalties	0	0	$ 0
Local Tax Authority
Income Tax Benefits [Line Items]
Valuation allowance - loss carryforward	2,000,000
Federal Tax Authority | Capital Loss Carryforward
Income Tax Benefits [Line Items]
Valuation allowance - general business credits	3,000,000
CMS Energy
Income Tax Benefits [Line Items]
Increase (decrease) to deferred tax assets related to tax rate changes		239,000,000
Tax Years 2008-2011 | State Tax Authority
Income Tax Benefits [Line Items]
Proceeds from income tax refunds	$ 1,000,000
Income taxes, net | Consumers Energy Company
Income Tax Benefits [Line Items]
Net regulatory tax liability		$ 1,600,000,000