Only recently did document editors become available online, but document editors in a general sense have been available offline in some form since the late sixties.
QED was a
line-oriented text editor written in 1965, a later version of which was used at Bell Labs.
O26 was written for the CDC 6000 series mainframe computer in 1967.
Vi was a
screen-oriented text editor written for Unix in 1976 and which was later outpaced in popularity by
VIM (or Vi IMproved), a command-line or GUI-based interface for editing text.
On the document editor side of things, when Microsoft released its flagship document editor program Microsoft Word in 1983, there were plenty of so-called visual or full-screen editor competitors such as WordStar (released 1978), WordPerfect (released 1979), Multimate (released 1982), Samna (released 1984), and MacWrite (released 1984). Word innovated relative to DOS applications by supporting the use of a mouse \cite{wiki:xxx}.
Today, Microsoft Word is the dominant document editor globally with some 1.2 billion users worldwide \cite{msft} while Google comes in second with more than 240M users \cite{tnw}. But Microsoft wasn't always the dominant document editor -- and only recently did Office 365 move online.
Philosophy of the document editor and WYSIWYG
The modern document editor more or less mirrors the pen-and-paper tools of the pre-computing age. Google Docs and Microsoft Word present to the user a workspace that is intended to mimic the look and feel of a physical piece of paper.