Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets
Restricted cash equivalents	$ 17	$ 17
Derivative instruments	1	1
Liabilities
Derivative instruments	17	8
Consumers Energy Company
Assets
Restricted cash equivalents	15	17
Derivative instruments	1	1
Liabilities
Derivative instruments	0	0
Fair Value, Inputs, Level 1
Assets
Restricted cash equivalents	17	17
Nonqualified deferred compensation plan assets	23	18
Liabilities
Nonqualified deferred compensation plan liabilities	23	18
Fair Value, Inputs, Level 1 | Consumers Energy Company
Assets
Restricted cash equivalents	15	17
Nonqualified deferred compensation plan assets	18	14
Liabilities
Nonqualified deferred compensation plan liabilities	18	14
Fair Value, Inputs, Level 1, 2 and 3
Assets
Total assets	41	36
Liabilities
Total liabilities	40	26
Fair Value, Inputs, Level 1, 2 and 3 | Consumers Energy Company
Assets
Total assets	34	33
Liabilities
Total liabilities	18	14
Common Stock | Fair Value, Inputs, Level 1 | Consumers Energy Company
Assets
CMS Energy common stock	$ 0	$ 1