Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended							3 Months Ended		9 Months Ended		12 Months Ended
	Jul. 31, 2017	Mar. 31, 2017	Feb. 28, 2017	Jan. 31, 2017	Sep. 30, 2016	Aug. 31, 2016	Mar. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016	Oct. 31, 2017
Public Utilities, General Disclosures [Line Items]
Regulatory liability current								$ 85		$ 85		$ 95
Accrued rate refunds								35		35		21
Revenue								1,527	$ 1,587	4,805	$ 4,759
Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory liability current								85		85		95
Accrued rate refunds								35		35		21
Revenue								1,437	$ 1,498	4,536	$ 4,514
2016 Electric Rate Case [Member] | Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Annual rate increase requested							$ 225
Rate of return on equity requested							10.70%
Annual rate increase self-implemented					$ 170
Annual rate increase authorized			$ 113
Rate of return on equity authorized			10.10%
2016 Electric Rate Case [Member] | Revenue Subject To Refund [Member] | Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory liability current								17		17
2016 Electric Rate Case [Member] | Revenue Subject To Refund [Member] | Consumers Energy Company [Member] | Subsequent Event [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory liability current													$ 17
Gas Rate Case [Member] | Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Annual rate increase requested						$ 90
Amended Annual Rate Increase Request		$ 80
Rate of return on equity requested						10.60%
Investment recovery, 2018	$ 18
Investment recovery, 2019	18
Annual rate increase self-implemented				$ 20
Annual rate increase authorized	$ 29
Energy Optimization [Member] | Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Authorized recovery/collection								$ 18		$ 18
Revenue												$ 18
Depreciation Rate Case [Member] | Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Annual rate increase authorized		$ (3)
FERC Transmission Order [Member] | Consumers Energy Company [Member] | Electric Transmission [Member]
Public Utilities, General Disclosures [Line Items]
Rate of return on equity authorized					10.32%	12.38%
Power Supply Cost Recover (PSCR) [Member] | Consumers Energy Company [Member] | Electric Transmission [Member]
Public Utilities, General Disclosures [Line Items]
Accrued rate refunds			$ 28