Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2021	Dec. 31, 2020
Assets
Restricted cash equivalents	$ 28	$ 17
Derivative instruments	0	1
Liabilities
Derivative instruments	14	17
Consumers Energy Company
Assets
Restricted cash equivalents	25	15
Derivative instruments	0	1
Liabilities
Derivative instruments	0	0
Fair Value, Inputs, Level 1
Assets
Restricted cash equivalents	28	17
Nonqualified deferred compensation plan assets	24	23
Liabilities
Nonqualified deferred compensation plan liabilities	24	23
Fair Value, Inputs, Level 1 | Consumers Energy Company
Assets
Restricted cash equivalents	25	15
Nonqualified deferred compensation plan assets	19	18
Liabilities
Nonqualified deferred compensation plan liabilities	19	18
Fair Value, Inputs, Level 1, 2 and 3
Assets
Total assets	52	41
Liabilities
Total liabilities	38	40
Fair Value, Inputs, Level 1, 2 and 3 | Consumers Energy Company
Assets
Total assets	44	34
Liabilities
Total liabilities	$ 19	$ 18