Pandas percentage of total with groupby

This is obviously simple, but as a numpy newbe I’m getting stuck.

I have a CSV file that contains 3 columns, the State, the Office ID, and the Sales for that office.

I want to calculate the percentage of sales per office in a given state (total of all percentages in each state is 100%).

df = pd.DataFrame({'state': ['CA', 'WA', 'CO', 'AZ'] * 3,
                   'office_id': range(1, 7) * 2,
                   'sales': [np.random.randint(100000, 999999)
                             for _ in range(12)]})

df.groupby(['state', 'office_id']).agg({'sales': 'sum'})

This returns:

                  sales
state office_id        
AZ    2          839507
      4          373917
      6          347225
CA    1          798585
      3          890850
      5          454423
CO    1          819975
      3          202969
      5          614011
WA    2          163942
      4          369858
      6          959285

I can’t seem to figure out how to “reach up” to the state level of the groupby to total up the sales for the entire state to calculate the fraction.