Financial Instruments (Summary of the Sales Activity for Investment Securities) (Details) (USD $)
In Millions
3 Months Ended9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Sep. 30, 2011
Sep. 30, 2010
Proceeds from Sales of Available-for-Sale Securities$ 1[1] $ 2[1]$ 1[1]
Gross Realized Losses from Sales of Available-for-Sale Securities Less Than1111
Consumers Energy Company [Member]
    
Proceeds from Sales of Available-for-Sale Securities1[1] 1[1] 
Gross Realized Losses from Sales of Available-for-Sale Securities Less Than$ 1$ 1$ 1$ 1
[1]1 All of the proceeds related to sales of investments that were held within the SERP and classified as available for sale. Realized losses on these sales were less than $1 million for both CMS Energy and Consumers during each period.