Revenue (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Uncollectable expense	$ 5	$ 6
Consumers Energy Company
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Unbilled receivables	$ 354		$ 426