Dataframe values are not converting to String properly [duplicate]

I am having data in excel and reading as below and i am trying to convert all data to string-

import pandas as pd
df=pd.read_excel('excel')
df.fillna('(Null)').astype(str)

even though i convert to string using astype(str), but in some case, i get float values as below(67.0) and how do i avoid this keeping just as string '67'-

  Name  id
0   A   123
1   B   67.0
2   C   30
3   D   40
4   E   50