Related Party Transactions - Consumers (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Purchased power - related parties	$ 87	$ 82	$ 85
Accounts payable - related parties	9	9
Investments	57	50
CMS Energy [Member]
Accounts payable - related parties	152	163
Investments	5,312	5,096
Consumers Energy Company [Member]
Purchased power - related parties	86	81	84
Accounts payable - related parties	11	11
Investment Owned, Balance, Shares	1.3
Investments	$ 32	$ 35