Income Taxes (Narrative) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Income Tax Benefits [Line Items]
Interest and penalties	$ 0.0	$ 0.0	$ 0.0
IRS audit, adjustments to income tax expense	0.0
Consumers Energy Company [Member]
Income Tax Benefits [Line Items]
Reduction of income tax expense	39.0	39.0	39.0
Interest and penalties	0.0	$ 0.0	$ 0.0
IRS audit, adjustments to income tax expense	$ 0.0