Financial Instruments (Schedule Of Investment Securities) (Details) - USD ($) $ in Millions		Sep. 30, 2018	Dec. 31, 2017
Held to maturity
Cost		$ 22	$ 16
Unrealized Gains		0	0
Unrealized Losses		(1)	0
Securities held to maturity		21	16
Common Stock | Consumers Energy Company
Available for sale
CMS Energy common stock cost			2
CMS Energy common stock - unrealized gains			19
CMS Energy Common Stock - unrealized losses			0
CMS Energy common stock - fair value			21
DB SERP
Available for sale
Cost		0	141
Unrealized Gains		0	0
Unrealized Losses		0	0
Fair Value	[1]	0	141
DB SERP | Consumers Energy Company
Available for sale
Cost		0	102
Unrealized Gains		0	0
Unrealized Losses		0	0
Fair Value	[1]	$ 0	102
Accounting Standards Update 2016-01 | Consumers Energy Company
Held to maturity
Cumulative effect of change in accounting principle			$ 19

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