Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Assets
DB SERP cash equivalents	$ 0	$ 1
Derivative instruments	0	1
Liabilities
Derivative instruments	4	3
Consumers Energy Company
Assets
DB SERP cash equivalents	0	0
Derivative instruments	0	1
Liabilities
Derivative instruments	0	0
Fair Value, Inputs, Level 1
Assets
Cash equivalents	20	27
Restricted cash and cash equivalents	33	21
Nonqualified deferred compensation plan assets	15	14
Liabilities
Nonqualified deferred compensation plan liabilities	15	14
Fair Value, Inputs, Level 1 | Consumers Energy Company
Assets
Cash equivalents	0	0
Restricted cash and cash equivalents	27	17
Nonqualified deferred compensation plan assets	12	10
Liabilities
Nonqualified deferred compensation plan liabilities	12	10
Fair Value, Inputs, Level 1 | DB SERP
Assets
DB SERP cash equivalents	0	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	68	64
Liabilities
Total	19	17
Fair Value, Inputs, Level 1, 2 and 3 | Consumers Energy Company
Assets
Total	40	29
Liabilities
Total	$ 12	$ 10