Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2021	Dec. 31, 2020
Assets
Restricted cash equivalents	$ 30	$ 17
Derivative instruments	2	1
Liabilities
Derivative instruments	9	11
Consumers Energy Company
Assets
Restricted cash equivalents	26	15
Derivative instruments	2	1
Liabilities
Derivative instruments	0	0
Fair Value, Inputs, Level 1
Assets
Restricted cash equivalents	30	17
Nonqualified deferred compensation plan assets	25	23
Liabilities
Nonqualified deferred compensation plan liabilities	25	23
Fair Value, Inputs, Level 1 | Consumers Energy Company
Assets
Restricted cash equivalents	26	15
Nonqualified deferred compensation plan assets	20	18
Liabilities
Nonqualified deferred compensation plan liabilities	20	18
Fair Value, Inputs, Level 1, 2 and 3
Assets
Total assets	57	41
Liabilities
Total liabilities	34	34
Fair Value, Inputs, Level 1, 2 and 3 | Consumers Energy Company
Assets
Total assets	48	34
Liabilities
Total liabilities	$ 20	$ 18