Consolidated Balance Sheets (Parenthetical) (USD $) In Millions, except Share data, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Allowances for doubtful accounts receivable	$ 31	$ 32
Allowances for doubtful notes receivable	5	5
Common stock, shares authorized	350,000,000	350,000,000
Common stock, shares outstanding	265,700,000	264,100,000
Consumers Energy Company [Member]
Allowances for doubtful accounts receivable	$ 29	$ 30
Common stock, shares authorized	125,000,000	125,000,000
Common stock, shares outstanding	84,100,000	84,100,000