Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Benefits [Line Items]
U.S. federal income tax rate		35.00%
Alternative minimum tax, recovery period		4 years
Increase (decrease) to deferred income tax expense related to tax rate changes		$ 148
Increase (decrease) to deferred tax liabilities related to tax rate changes		1,300
Valuation allowance		15	$ 5
Alternative minimum tax credit reclassification		124
Alternative minimum tax credits, sequestration		9
Interest and penalties		0	0	$ 0
General Business Tax Credit Carryforward [Member]
Income Tax Benefits [Line Items]
Valuation Allowance - General business credits		$ 3
Tax Year 2018 [Member]
Income Tax Benefits [Line Items]
Alternative minimum tax, recovery period		4 years
Threshold of taxable income for NOL		80.00%
Percent of cost of property that may be expensed when placed in service		100.00%
Threshold of adjusted taxable income fore net interest expense deduction		30.00%
Scenario, Forecast [Member]
Income Tax Benefits [Line Items]
U.S. federal income tax rate	21.00%
Cash Refunds Of Alternative Minimum Tax Credits [Member]
Income Tax Benefits [Line Items]
Valuation allowance		$ 10
CMS Energy [Member]
Income Tax Benefits [Line Items]
Increase (decrease) to deferred tax assets related to tax rate changes		239
Consumers Energy Company [Member]
Income Tax Benefits [Line Items]
Increase (decrease) to deferred income tax expense related to tax rate changes		33
Interest and penalties		0	$ 0	$ 0
Local Jurisdiction [Member]
Income Tax Benefits [Line Items]
Valuation Allowance - Loss carryforward		$ 2