Earnings Per Share - CMS Energy (Impact on the Calulation of EPS if Using the If-Converted Method) (Details) (Convertible Debt Securities [Member], USD $)
In Millions, unless otherwise specified
9 Months Ended
Sep. 30, 2011
Sep. 30, 2010
Convertible Debt Securities [Member]
  
Debt Instrument, Interest Rate, Stated Percentage7.75%7.75%
Potential impact of antidilutive debentures - income$ 1.0$ 1.0
Potential impact of antidilutive debentures - shares0.70.7