Income Taxes (Principal Components Of Deferred Income Tax Assets And Liabilities) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Tax loss and credit carryforwards	$ 453	$ 871
Net regulatory tax liability	411	27
Reserves and accruals	40	69
Total deferred income tax assets	904	967
Valuation allowance	(15)	(5)
Total deferred income tax assets, net of valuation reserves	889	962
Plant, property, and equipment	(1,891)	(2,902)
Employee benefits	(96)	(158)
Securitized costs	(71)	(118)
Gas inventory	(37)	(65)
Other	(63)	(6)
Total deferred income tax liabilities	(2,158)	(3,249)
Total net deferred income tax liabilities	(1,269)	(2,287)
Consumers Energy Company [Member]
Tax loss and credit carryforwards	101	190
Net regulatory tax liability	411	27
Reserves and accruals	21	37
Total deferred income tax assets, net of valuation reserves	533	254
Plant, property, and equipment	(1,901)	(2,924)
Employee benefits	(105)	(181)
Securitized costs	(71)	(118)
Gas inventory	(37)	(65)
Other	(59)	(8)
Total deferred income tax liabilities	(2,173)	(3,296)
Total net deferred income tax liabilities	$ (1,640)	$ (3,042)