If column B is empty then put tthe formul
=if(A1="Header","Header","Detail") in B1.
Copy it down to the bottom of your data.
Column B should look like what you want, if I understand properly.
If Column B is already in use, insert a new column B and use that.
If the result is what you want, copy the whole of Column B. Then us
Edit>Paste Special and Values and paste it right back on top o
itself.