Fair Value Measurements (Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Assets
Cash equivalents	[1]	$ 139	$ 74
Restricted cash and cash equivalents	[1]	42	17
Nonqualified deferred compensation plan assets	[1]	14	14
DB SERP
Cash equivalents	[1]	1	5
Derivative instruments
Commodity contracts	[1]	1	1
Total	[1]	197	252
Liabilities
Nonqualified deferred compensation plan liabilities	[1]	14	14
Derivative instruments
Commodity contracts	[1]	1	1
Total	[1]	15	15
Consumers Energy Company
Assets
Cash equivalents	[1]	0	0
Restricted cash and cash equivalents	[1]	28	17
Nonqualified deferred compensation plan assets	[1]	11	10
DB SERP
Cash equivalents		1	4
Derivative instruments
Commodity contracts	[1]	1	1
Total	[1]	42	155
Liabilities
Nonqualified deferred compensation plan liabilities	[1]	11	10
Derivative instruments
Commodity contracts	[1]	1	0
Total	[1]	12	10
DB SERP
DB SERP
Cash equivalents	[1]	1	5
Debt securities	[1]	0	141
DB SERP | Consumers Energy Company
DB SERP
Cash equivalents	[1]	1	4
Debt securities	[1]	0	102
Common Stock | Consumers Energy Company
Assets
CMS Energy common stock	[1]	$ 1	$ 21

[1] 1 All assets and liabilities were classified as Level 1 with the exception of some commodity contracts, which were classified as Level 3.