How to add - to the cell that has dates

I have a 'date' column that has a date value as 20170423 (yyyymmdd) how can i change it to 2017-04-23?

dataframe = df column name 'date'

I read the below post, but none of the solutions worked

Fastest way to insert these dashes in python string?

Example

d = df['Date'] 
df['Date'] = '%s-%s-%s' % (d[:4], d[4:6], d[6:])

The output 0 0 20170711\n1 20170718\n2 20170718\n3... when i exported in .csv