Financial Instruments (Narrative) (Details) - USD ($) $ in Millions		9 Months Ended
		Sep. 30, 2018	Jul. 31, 2018	Dec. 31, 2017
Financial Instruments [Line Items]
Portion of long-term debt supported by third-party credit enhancements		$ 35		$ 103
Consumers Energy Company
Financial Instruments [Line Items]
Portion of long-term debt supported by third-party credit enhancements		$ 35		103
DB SERP | Minimum
Financial Instruments [Line Items]
Debt security term		1 year
DB SERP | Maximum
Financial Instruments [Line Items]
Debt security term		10 years
CMS Energy Note Payable
Financial Instruments [Line Items]
Notes payable			$ 146
Interest Rate			4.10%
Common Stock | Consumers Energy Company
Financial Instruments [Line Items]
Equity investment	[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.