Contingencies And Commitments (Guarantees) (Details) $ in Millions		12 Months Ended
		Dec. 31, 2017 USD ($) $ / MWh MW	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Consumers Energy Company [Member]
Guarantees And Other Contingencies [Line Items]
Expiration Date	[1]	Indefinite
Maximum obligation	[1]	$ 30
Guarantees [Member]
Guarantees And Other Contingencies [Line Items]
Expiration Date	[1]	Indefinite
Maximum obligation	[1]	$ 45
Indemnity Obligations From Stock And Asset Sales Agreements [Member]
Guarantees And Other Contingencies [Line Items]
Expiration Date	[2]	Indefinite
Maximum obligation	[2]	$ 153
Carrying Amount	[2]	7
Tax And Other Indemnity Obligations [Member] | Consumers Energy Company [Member]
Guarantees And Other Contingencies [Line Items]
Carrying Amount		1
MCV PPA [Member] | Consumers Energy Company [Member]
Guarantees And Other Contingencies [Line Items]
Purchase Obligations Total		2,621
Purchase Obligations Due in Year One		350
Purchase Obligations Due in Year Two		348
Purchase Obligations Due in Year Three		346
Purchase Obligations Due in Year Four		335
Purchase Obligations Due in Year Five		339
Purchase Obligations Due After Year Five		$ 903
Term of Unrecorded Unconditional Purchase Obligations		35 years
Unrecorded Unconditional Purchase Obligations, Minimum Quantity Required | MW		1,240
Unrecorded Unconditional Purchase Obligation Capacity Charge Per Mwh | $ / MWh		10.14
Unrecorded Unconditional Purchase Obligation, Annual Contribution to Renewable Resources Program By Counterparty		$ 5
Unrecorded Unconditional Purchase Obligation, Contract Extension Period		5 years
Unrecorded Unconditional Purchase Obligation, Purchases		$ 321	$ 305	$ 282
Palisades PPA [Member]
Guarantees And Other Contingencies [Line Items]
Unrecorded Unconditional Purchase Obligations, Maximum Quantity | MW		798
Palisades PPA [Member] | Consumers Energy Company [Member]
Guarantees And Other Contingencies [Line Items]
Purchase Obligations Total		$ 1,647
Purchase Obligations Due in Year One		367
Purchase Obligations Due in Year Two		378
Purchase Obligations Due in Year Three		388
Purchase Obligations Due in Year Four		400
Purchase Obligations Due in Year Five		114
Unrecorded Unconditional Purchase Obligation, Purchases		$ 366	363	352
Unrecorded Unconditional Purchase Obligations, Maximum Quantity | MW		798
Related Parties PPA [Member] | Consumers Energy Company [Member]
Guarantees And Other Contingencies [Line Items]
Purchase Obligations Total		$ 1,546
Purchase Obligations Due in Year One		87
Purchase Obligations Due in Year Two		87
Purchase Obligations Due in Year Three		94
Purchase Obligations Due in Year Four		96
Purchase Obligations Due in Year Five		100
Purchase Obligations Due After Year Five		1,082
Other PPA [Member] | Consumers Energy Company [Member]
Guarantees And Other Contingencies [Line Items]
Purchase Obligations Total		3,345
Purchase Obligations Due in Year One		238
Purchase Obligations Due in Year Two		235
Purchase Obligations Due in Year Three		236
Purchase Obligations Due in Year Four		232
Purchase Obligations Due in Year Five		242
Purchase Obligations Due After Year Five		2,162
Unrecorded Unconditional Purchase Obligation, Purchases		349	$ 348	$ 347
PPA [Member] | Consumers Energy Company [Member]
Guarantees And Other Contingencies [Line Items]
Purchase Obligations Total		9,159
Purchase Obligations Due in Year One		1,042
Purchase Obligations Due in Year Two		1,048
Purchase Obligations Due in Year Three		1,064
Purchase Obligations Due in Year Four		1,063
Purchase Obligations Due in Year Five		795
Purchase Obligations Due After Year Five		4,147
Other Purchase Obligations [Member]
Guarantees And Other Contingencies [Line Items]
Purchase Obligations Total		2,026
Purchase Obligations Due in Year One		891
Purchase Obligations Due in Year Two		541
Purchase Obligations Due in Year Three		186
Purchase Obligations Due in Year Four		61
Purchase Obligations Due in Year Five		56
Purchase Obligations Due After Year Five		291
Other Purchase Obligations [Member] | Consumers Energy Company [Member]
Guarantees And Other Contingencies [Line Items]
Purchase Obligations Total		1,787
Purchase Obligations Due in Year One		859
Purchase Obligations Due in Year Two		511
Purchase Obligations Due in Year Three		156
Purchase Obligations Due in Year Four		48
Purchase Obligations Due in Year Five		44
Purchase Obligations Due After Year Five		$ 169

[1] At Consumers, this obligation comprises a guarantee provided to the U.S. Department of Energy in connection with a settlement agreement regarding damages resulting from the department's failure to accept spent nuclear fuel from nuclear power plants formerly owned by Consumers. At CMS Energy, the guarantee obligations comprise Consumers' guarantee to the U.S. Department of Energy and CMS Energy's 1994 guarantee of non-recourse revenue bonds issued by Genesee. For additional details on this guarantee, see Note 20, Variable Interest Entities.

[2] These obligations arose from stock and asset sale agreements under which CMS Energy or a subsidiary of CMS Energy indemnified the purchaser for losses resulting from various matters, primarily claims related to taxes. CMS Energy believes the likelihood of material loss to be remote for the indemnity obligations not recorded as liabilities.