Revenue (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Uncollectable expense	$ 6	$ 7
Unbilled receivables	341		$ 409
Consumers Energy Company
Revenue, Initial Application Period Cumulative Effect Transition [Line Items]
Uncollectable expense	6	$ 7
Unbilled receivables	$ 341		$ 409