Today, the SEC held a public meeting in which the staff of the SEC proposed a rule and the commission approved a proposed rule which would mandate the use of XBRL for filings with the SEC. The mandate would be phased in.
The following link to an FEI blog post which explains the rule.
And this is a link to an archive of the actual public meeting the SEC held in which it approved the proposed rule.
The proposed rule will be published in 7 to 10 days from what I hear. There will then be a 60 day public comment period.