Bowne & Co's Rob Blake has prepared a new white paper that explains XBRL in clear terms and also traces its history from its inception to the most recent SEC announcements making it mandatory. This is an essential read for those interested in knowing what makes XBRL tick.
http://www.accountingweb.com/whitepapers/XBRL_The-Dawn-of-a-New-EDGAR--Bowne.pdf