Income Taxes (Reconciliation Of Beginning And Ending Uncertain Tax Benefits) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Balance at beginning of period	$ 5	$ 6	$ 5
Additions for current year tax positions	10		1
Additions for prior year tax positions			1
Reductions for prior year tax positions	(1)		(1)
Settlements		(1)
Balance at end of period	14	5	6
Consumers Energy Company [Member]
Balance at beginning of period	5	6	5
Additions for current year tax positions	17		1
Additions for prior year tax positions			1
Reductions for prior year tax positions	(1)		(1)
Settlements		(1)
Balance at end of period	$ 21	$ 5	$ 6