Add .idea directory to the list of ignored files
First, add it to .gitignore, so it is not accidentally committed by you (or someone else) again:
.idea
Remove it from repository
Second, remove the directory only from the repository, but do not delete it locally. To achieve that, do what is listed here:
Remove a file from a Git repository without deleting it from the local filesystem
Send the change to others
Third, commit the .gitignore file and the removal of .idea from the repository. After that push it to the remote(s).
Summary
The full process would look like this:
$ echo '.idea' >> .gitignore
$ git rm -r --cached .idea
$ git add .gitignore
$ git commit -m '(some message stating you added .idea to ignored entries)'
$ git push
(optionally you can replace last line with git push some_remote, where some_remote is the name of the remote you want to push to)