Consolidated Statements Of Comprehensive Income (Parenthetical) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Retirement benefits liability adjustments, tax expense (tax benefit)
Unrealized gain (loss) on investments, tax expense (tax benefit)
Consumers Energy Company [Member]
Retirement benefits liability adjustments, tax expense (tax benefit)
Unrealized gain (loss) on investments, tax expense (tax benefit)		(2)
Reclassification adjustments included in net income, tax	$ (1)