When the SEC mandated XBRL filing late last year, the commission made clear that only the largest public companies would be required to comply in the beginning. The 500 largest companies that file using U.S. Generally Acceptable Accounting Principles and have fiscal years ending on or after June 15, 2009. The rule doesn't phase in all public companies until June 15, 2011, but as many experts have said, the XBRL tagging process is not one that can be accomplished overnight.
The SEC's compliance guide explains the rule in simpler terms and then lays out the list of forms to which the XBRL rule applies, as well as explaining where to find the applicable list of tags that must be used. The guide also explains the early compliance option that is available, as well as the penalties for non-compliance. Perhaps of most value, however, is information near the end of the guide. It's a linked list of resources companies may use as they prepare to meet their respective deadlines.