Income Taxes (Schedule Of Effective Income Rate Reconciliation) (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Dec. 31, 2020
Income Taxes [Line Items]
U.S. federal income tax rate	21.00%	21.00%
Increase (decrease) in income taxes from:
State and local income taxes, net of federal effect	5.10%	4.60%
TCJA excess deferred taxes	(5.30%)	(3.90%)
Production tax credits	(4.70%)	(2.80%)
Accelerated flow-through of regulatory tax benefits	(3.00%)	(1.50%)
Research and development tax credits, net	(0.30%)	(3.40%)
Refund of alternative minimum tax sequestration	0.00%	(3.30%)
Other, net	0.20%	(0.70%)
Effective tax rate	13.00%	10.00%
Refund of alternative minimum tax sequestration			$ (9)
Consumers Energy Company
Income Taxes [Line Items]
U.S. federal income tax rate	21.00%	21.00%
Increase (decrease) in income taxes from:
State and local income taxes, net of federal effect	5.10%	4.90%
TCJA excess deferred taxes	(5.10%)	(3.40%)
Production tax credits	(2.60%)	(1.40%)
Accelerated flow-through of regulatory tax benefits	(3.70%)	(1.90%)
Research and development tax credits, net	(0.20%)	(3.10%)
Other, net	(0.20%)	(0.30%)
Effective tax rate	14.30%	15.80%
Research Tax Credit Carryforward
Increase (decrease) in income taxes from:
Increase (decrease) in tax credit carryforward			9
Research Tax Credit Carryforward | Consumers Energy Company
Increase (decrease) in income taxes from:
Increase (decrease) in tax credit carryforward			$ 8