Retirement Benefits (Schedule Of Net Periodic Benefit Cost Not yet Recognized) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Total regulatory asset	$ 1,784	$ 2,108
Total regulatory liabilities	(3,795)	(2,136)
DB Pension Plans And DB SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets (liabilities), Net loss	1,017	1,062
Regulatory assets (liabilities), Prior service cost (credit)	11	15
Total regulatory asset	1,028	1,077
AOCI, Net loss (gain)	97	93
AOCI, Prior service cost (credit)	1	1
Total amounts recognized in regulatory assests (liabilities) and AOCI	1,126	1,171
DB Pension Plans And DB SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets (liabilities), Net loss	1,017	1,062
Regulatory assets (liabilities), Prior service cost (credit)	11	15
Total regulatory asset	1,028	1,077
AOCI, Net loss (gain)	36	33
Total amounts recognized in regulatory assests (liabilities) and AOCI	1,064	1,110
OPEB Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets (liabilities), Net loss	316	483
Regulatory assets (liabilities), Prior service cost (credit)	(451)	(187)
Total regulatory asset		296
Total regulatory liabilities	(135)
AOCI, Net loss (gain)	(6)	(8)
AOCI, Prior service cost (credit)	(12)	(6)
Total amounts recognized in regulatory assests (liabilities) and AOCI	(153)	282
OPEB Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Regulatory assets (liabilities), Net loss	316	483
Regulatory assets (liabilities), Prior service cost (credit)	(451)	(187)
Total regulatory asset		296
Total regulatory liabilities	(135)
Total amounts recognized in regulatory assests (liabilities) and AOCI	$ (135)	$ 296