You might be interested in looking at, or contributing to, our MicroXBRL wiki.
There have been discussions about rendering around the XBRL consortium for many years and now, inside the consortium, there is an effort to capture them. The problem, from our perspective, is that accountants and report preparers need to be able to make their reports look exactly the way *they* want them to. Providing an XBRL data file together with a file containing a range of rendering metadata isn’t terribly attractive either to the preparer, or to the software vendors that have to support them. So instead, drawing on the ideas of the folk over at Microformats.org as well as the SVG
group (including Jonathan Watt) we are experimenting with a much more direct approach. Publishing XHTML files that render the report the way the user wants, and embedding XBRL tags right inside those files. It’s simple. It meets the major user requirements. It can probably be extended to allow fancier requirements to be met as well. Your input is welcome!