pandas groupby mean with nan

I have the following dataframe:

date id  cars
2012 1    4  
2013 1    6
2014 1    NaN    
2012 2    10 
2013 2    20 
2014 2    NaN  

Now, I want to get the mean of cars over the years for each id ignoring the NaN's. The result should be like this:

date id  cars  result
2012 1    4      5
2013 1    6      5
2014 1    NaN    5
2012 2    10     15
2013 2    20     15
2014 2    NaN    15

I have the following command:

df["result"]=df.groupby("id")["cars"].mean()

The command runs without errors, but the result column only has NaN's. What did I do wrong?