Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2020	Dec. 31, 2019
Assets
Derivative instruments	$ 2	$ 1
Liabilities
Derivative instruments	20	8
Consumers Energy Company
Assets
Derivative instruments	2	1
Liabilities
Derivative instruments	1	0
Fair Value, Inputs, Level 1
Assets
Restricted cash equivalents	39	17
Nonqualified deferred compensation plan assets	20	18
Liabilities
Nonqualified deferred compensation plan liabilities	20	18
Fair Value, Inputs, Level 1 | Consumers Energy Company
Assets
Restricted cash equivalents	24	17
Nonqualified deferred compensation plan assets	16	14
Liabilities
Nonqualified deferred compensation plan liabilities	16	14
Fair Value, Inputs, Level 1 | Common Stock | Consumers Energy Company
Assets
CMS Energy common stock	1	1
Fair Value, Inputs, Level 1, 2 and 3
Assets
Total	61	36
Liabilities
Total	40	26
Fair Value, Inputs, Level 1, 2 and 3 | Consumers Energy Company
Assets
Total	43	33
Liabilities
Total	$ 17	$ 14