Retirement Benefits (Schedule Of Net Periodic Benefit Cost Not yet Recognized) (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015
Pension And DB SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets (liabilities), Net loss	$ 1,062	$ 944
Regulatory assets (liabilities), Prior service cost (credit)	15	19
Regulatory assets (liabilities)	1,077	963
AOCI, Net loss (gain)	93	86
AOCI, Prior service cost (credit)	1	1
Total amounts not yet recognized as component of net periodic benefit cost	1,171	1,050
Pension And DB SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets (liabilities), Net loss	1,062	944
Regulatory assets (liabilities), Prior service cost (credit)	15	19
Regulatory assets (liabilities)	1,077	963
AOCI, Net loss (gain)	33	29
Total amounts not yet recognized as component of net periodic benefit cost	1,110	992
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets (liabilities), Net loss	483	360
Regulatory assets (liabilities), Prior service cost (credit)	(187)	(227)
Regulatory assets (liabilities)	296	133
AOCI, Net loss (gain)	(8)	(11)
AOCI, Prior service cost (credit)	(6)	(8)
Total amounts not yet recognized as component of net periodic benefit cost	282	114
OPEB [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets (liabilities), Net loss	483	360
Regulatory assets (liabilities), Prior service cost (credit)	(187)	(227)
Regulatory assets (liabilities)	296	133
Total amounts not yet recognized as component of net periodic benefit cost	$ 296	$ 133