Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions	Sep. 30, 2019	Dec. 31, 2018
Assets
Restricted cash and cash equivalents	$ 29	$ 21
DB SERP cash equivalents	1	1
Derivative instruments	2	1
Liabilities
Derivative instruments	11	3
Consumers Energy Company
Assets
Restricted cash and cash equivalents	25	17
Derivative instruments	2	1
Liabilities
Derivative instruments	1	0
Fair Value, Inputs, Level 1
Assets
Cash equivalents	0	27
Restricted cash and cash equivalents	29	21
Nonqualified deferred compensation plan assets	17	14
Liabilities
Nonqualified deferred compensation plan liabilities	17	14
Fair Value, Inputs, Level 1 | Consumers Energy Company
Assets
Cash equivalents	0	0
Restricted cash and cash equivalents	25	17
Nonqualified deferred compensation plan assets	13	10
Liabilities
Nonqualified deferred compensation plan liabilities	13	10
Fair Value, Inputs, Level 1 | DB SERP
Assets
DB SERP cash equivalents	1	1
Fair Value, Inputs, Level 1 | DB SERP | Consumers Energy Company
Assets
DB SERP cash equivalents	0	0
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	49	64
Liabilities
Total	28	17
Fair Value, Inputs, Level 1, 2 and 3 | Consumers Energy Company
Assets
Total	41	29
Liabilities
Total	$ 14	$ 10