Financial Instruments (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Financial Instruments [Line Items]
Portion of long-term debt supported by third-party credit enhancements	$ 35	$ 35
Consumers Energy Company
Financial Instruments [Line Items]
Reclassified gain net of tax	$ 0	(1)	$ 9
DB SERP
Financial Instruments [Line Items]
Gross reclassified gain			2
Reclassified gain net of tax			1
DB SERP | Consumers Energy Company
Financial Instruments [Line Items]
Gross reclassified gain			2
Reclassified gain net of tax			$ 1
CMS Energy Note Payable
Financial Instruments [Line Items]
Notes payable		$ 146
Interest rate		4.10%