Reportable Segments (Details) - USD ($) $ in Millions		3 Months Ended			9 Months Ended
		Sep. 30, 2018		Sep. 30, 2017	Sep. 30, 2018		Sep. 30, 2017	Dec. 31, 2017
Segment Reporting Information [Line Items]
Operating Revenue		$ 1,599		$ 1,527	$ 5,044		$ 4,805
Net income (loss) available to common stockholders		169		172	549		463
Plant, property, and equipment, gross		23,751			23,751			$ 22,506
Total Assets		23,913			23,913			23,050
Consumers Energy Company
Segment Reporting Information [Line Items]
Operating Revenue		1,502		1,437	4,752		4,536
Net income (loss) available to common stockholder		180		181	573		495
Plant, property, and equipment, gross		23,322			23,322			22,318
Total Assets		21,492			21,492			21,099
Operating Segments | Electric Utility
Segment Reporting Information [Line Items]
Operating Revenue		1,313		1,247	3,480		3,360
Net income (loss) available to common stockholders		199		176	468		394
Plant, property, and equipment, gross	[1]	15,772			15,772			15,221
Total Assets	[1]	13,850			13,850			13,906
Operating Segments | Electric Utility | Consumers Energy Company
Segment Reporting Information [Line Items]
Operating Revenue		1,313		1,247	3,480		3,360
Net income (loss) available to common stockholder		199		176	468		394
Plant, property, and equipment, gross	[1]	15,772			15,772			15,221
Total Assets	[1]	13,914			13,914			13,907
Operating Segments | Gas Utility
Segment Reporting Information [Line Items]
Operating Revenue		189		190	1,272		1,176
Net income (loss) available to common stockholders		(19)		5	105		101
Plant, property, and equipment, gross	[1]	7,534			7,534			7,080
Total Assets	[1]	7,513			7,513			7,139
Operating Segments | Gas Utility | Consumers Energy Company
Segment Reporting Information [Line Items]
Operating Revenue		189		190	1,272		1,176
Net income (loss) available to common stockholder		(19)		5	105		101
Plant, property, and equipment, gross	[1]	7,534			7,534			7,080
Total Assets	[1]	7,556			7,556			7,139
Operating Segments | Enterprises
Segment Reporting Information [Line Items]
Operating Revenue		57	[2]	58	181	[2]	172
Net income (loss) available to common stockholders		4		8	33		27
Plant, property, and equipment, gross		405			405			167
Total Assets		538			538			342
Other reconciling items
Segment Reporting Information [Line Items]
Operating Revenue		40	[3]	32	111	[3]	97
Net income (loss) available to common stockholders		(15)		$ (17)	(57)		$ (59)
Plant, property, and equipment, gross		40			40			38
Total Assets		2,012			2,012			1,663
Other reconciling items | Consumers Energy Company
Segment Reporting Information [Line Items]
Operating Revenue		0			0
Plant, property, and equipment, gross		16			16			17
Total Assets		$ 22			$ 22			$ 53

[1] Amounts include a portion of Consumers’ other common assets attributable to both the electric and gas utility businesses.

[2] Amounts represent the enterprises segment’s operating revenue from independent power production and CMS ERM’s sales of energy commodities in support of the independent power production portfolio.

[3] Amount represents EnerBank’s operating revenue from unsecured consumer installment loans for financing home improvements.