Consolidated Statements Of Comprehensive Income (Parenthetical) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net gain (loss) arising during the period, tax	$ (4)	$ (5)
Prior service credit adjustment, tax	3
Amortization of net actuarial loss, tax	1		$ 4
Amortization of prior service credit, tax
Unrealized gain (loss) on investments, tax expense (tax benefit)			(1)
Other-than-temporary impairment, tax		2
Consumers Energy Company [Member]
Net gain (loss) arising during the period, tax	(1)	(1)	2
Amortization of net actuarial loss, tax			2
Unrealized gain (loss) on investments, tax expense (tax benefit)	1	2	(1)
Reclassification adjustments included in net income, tax	$ (6)		$ (3)
Other-than-temporary impairment, tax		$ 2