Not sure whether this is a bug or whether you'd do anything to fix it, I thought mentioning it nevertheless.
Somehow I end up with some html/utf-8 files that have both LF and CRLF line endings.
Idea editor cannot handle the line endings change unless all the content is LF or CRLF (actually I am not sure if the editor is direction biased as didn't test all combinations).
For instance, I have this file, the file utility reports: "HTML document, UTF-8 Unicode text, with very long lines, with CRLF, LF line terminators"
I ask idea to convert it to LF, the line separator cue changes below the editor but when inspecting the file with same tool it reports back with same message above.
It seems to me that all direction line endings conversions fail with such a content.
I have tested this on Linux Mint 17.3, all files utf-8 (html)