Regulatory Matters (Narrative) (Details) - USD ($) $ in Millions	1 Months Ended						3 Months Ended		12 Months Ended
	Mar. 31, 2018	Feb. 28, 2018	Jan. 31, 2018	Oct. 31, 2017	Sep. 30, 2017	Mar. 31, 2017	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Public Utilities, General Disclosures [Line Items]
Regulatory liability current	$ 134						$ 134		$ 80
Revenue							1,953	$ 1,829
Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory liability current	134						134		80
Revenue							1,855	$ 1,737
Revenue Subject to Refund - Tax Reform Rate Change [Member] | Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory liability current	51						51
Electric Rate Case [Member] | Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Annual rate increase requested						$ 173
Annual rate increase requested, amended					$ 148
Rate of return on equity requested						10.50%
Annual rate increase self-implemented				$ 130
Annual rate increase authorized	$ 66
Rate of return on equity authorized	10.00%
Electric Rate Case [Member] | Revenue Subject to Refund - Other [Member] | Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Regulatory liability current	$ 36						36
Gas Rate Case [Member] | Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Recommended disallowance of cost recovery		$ (115)
Electric Rate Case Tax Reform Rate Change [Member] | Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Estimated change in rate			$ (116)
Gas Rate Case Tax Reform Rate Change [Member] | Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Annual rate increase requested	(48)
Estimated change in rate			$ (49)
Energy Waste Reduction Plan [Member] | Consumers Energy Company [Member]
Public Utilities, General Disclosures [Line Items]
Requested recovery/collection	$ 31						$ 31
Revenue									$ 31