Retirement Benefits (Schedule Of Retirement Benefit Plan Assets (Liabilities)) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Non-current liabilities	$ 766	$ 789
Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Non-current liabilities	711	730
DB Pension Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Non-current assets	143
Non-current liabilities	618	461
DB Pension Plans [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Current liabilities	147
Non-current liabilities	602	441
DB SERP [Member]
Defined Benefit Plan Disclosure [Line Items]
Current liabilities	9	8
Non-current liabilities	145	143
DB SERP [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Current liabilities	7	5
Non-current liabilities	105	104
OPEB Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Non-current assets	323
Non-current liabilities		144
OPEB Plan [Member] | Consumers Energy Company [Member]
Defined Benefit Plan Disclosure [Line Items]
Non-current assets	$ 276
Non-current liabilities		$ 181