I have a pandas series.groupby objects, call it data. If I print out the elements, it looks like this:

<pandas.core.groupby.generic.SeriesGroupBy object at ***>
(1, 0     397.44
    1     12.72
    2     422.40
Name: value, dtype: float64)
(2, 3     398.88
    4     6.48
    5     413.52
Name: value, dtype: float64)
(3, 6     398.40
    7     68.40
    8     18.96
    9     56.64
    10    406.56
Name: value, dtype: float64)
(4, 11    398.64
    12    14.64
    13    413.76
Name: value, dtype: float64)
...

I want to make an equivalent object, where the entries are the cumulative sum of each sublist in the series, minus the first entry of that list. I can get the cumulative sum easily enough using apply:

cumulative_sums = data.apply(lambda x: x.cumsum())

but when I try to subtract the first element of the list in what I would think of as the intuitive way (lambda x: x.cumsum()-x[0]) , I get a KeyError. How can I achieve what I am trying to do?