Retirement Benefits (Schedule of SERP Trust Assets, ABO and Contributions) (Details) - DB SERP - USD ($) $ in Millions	Dec. 31, 2023	Dec. 31, 2022
Defined Benefit Plan Disclosure [Line Items]
ABO	$ 115	$ 118
Consumers Energy Company
Defined Benefit Plan Disclosure [Line Items]
ABO	83	85
Trust assets
Defined Benefit Plan Disclosure [Line Items]
Trust assets	132	137
Trust assets | Consumers Energy Company
Defined Benefit Plan Disclosure [Line Items]
Trust assets	$ 98	$ 101