Related Party Transactions - Consumers (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Purchased power - related parties	$ 86	$ 86	$ 83
Accounts receivable - related parties	12	12
Consumers Energy Company [Member]
Related Party Transaction [Line Items]
Purchased power - related parties	90	88	$ 83
Accounts receivable - related parties	2	9
Due to related parties	27	24
Accounts receivable - related parties	2	9
Investments	21	$ 33
Consumers Energy Company [Member] | Credit Agreement [Member]
Related Party Transaction [Line Items]
Maximum borrowing capacity	300
Amount outstanding	$ 0