First, many people perceive documentation as something they write and shove in a pile. This isn't true, especially in regards to IT. Stuff changes and with it the documentation must be kept up to date. Yes, you have to invest the initial time, but once you do, revising the documentation in the future is a fairly easy process that should be done on a regular basis.
Documentation should always cover absolute worst case scenario. As in should the entire department be unavailable or turned to zombies or whatever and new people have to come in from the ground up.
And if you think there is a such thing as too much documentation, try complying with documentation for ISO... that can be a headache...
But @networknerd stated, the best way is to start with a template. Then outline what all you want to cover. It gets easier the more you do it.