Financial Instruments (Narrative) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Financial Instruments [Line Items]
Portion of long-term debt supported by third-party credit enhancements	$ 103	$ 103
Consumers Energy Company [Member]
Financial Instruments [Line Items]
Portion of long-term debt supported by third-party credit enhancements	$ 103	$ 103
DB SERP [Member] | Minimum [Member]
Financial Instruments [Line Items]
Debt security term	1 year
DB SERP [Member] | Maximum [Member]
Financial Instruments [Line Items]
Debt security term	10 years