Retirement Benefits (Schedule Of Net Periodic Benefit Cost Not yet Recognized) (Details) - USD ($) $ in Millions	Dec. 31, 2015	Dec. 31, 2014
Pension And DB SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets (liabilities), Net loss	$ 944	$ 1,012
Regulatory assets (liabilities), Prior service cost (credit)	19	7
Regulatory assets (liabilities)	963	1,019
AOCI, Net loss (gain)	86	99
AOCI, Prior service cost (credit)	1	1
Total amounts not yet recognized as component of net periodic benefit cost	1,050	1,119
Pension And DB SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets (liabilities), Net loss	944	1,012
Regulatory assets (liabilities), Prior service cost (credit)	19	7
Regulatory assets (liabilities)	963	1,019
AOCI, Net loss (gain)	29	39
Total amounts not yet recognized as component of net periodic benefit cost	992	1,058
OPEB [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets (liabilities), Net loss	360	419
Regulatory assets (liabilities), Prior service cost (credit)	(227)	(243)
Regulatory assets (liabilities)	133	176
AOCI, Net loss (gain)	(11)	(18)
AOCI, Prior service cost (credit)	(8)	(8)
Total amounts not yet recognized as component of net periodic benefit cost	114	150
OPEB [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets (liabilities), Net loss	360	419
Regulatory assets (liabilities), Prior service cost (credit)	(227)	(243)
Regulatory assets (liabilities)	133	176
Total amounts not yet recognized as component of net periodic benefit cost	$ 133	$ 176