Financial Instruments (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jul. 31, 2018	Dec. 31, 2015
Consumers Energy Company
Financial Instruments [Line Items]
Portion of long-term debt supported by third-party credit enhancements	$ 35	$ 103
Reclassified gain net of tax	(1)	9	$ 0
Gain on CMS Energy common stock		14
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%
US Treasury Securities | DB SERP | Minimum
Financial Instruments [Line Items]
Debt securities term		1 year
US Treasury Securities | DB SERP | Maximum
Financial Instruments [Line Items]
Debt securities term		10 years
Accumulated Deficit
Financial Instruments [Line Items]
Cumulative effect of change in accounting principle		$ 8	0		$ 33
Accumulated Deficit | Consumers Energy Company
Financial Instruments [Line Items]
Cumulative effect of change in accounting principle		19	$ 0		$ 0
Accumulated Deficit | Accounting Standards Update 2016-01 | Consumers Energy Company
Financial Instruments [Line Items]
Cumulative effect of change in accounting principle		19
Common Stock | Consumers Energy Company
Financial Instruments [Line Items]
CMS Energy common stock	$ 1	$ 21