The second phase of filing, in which companies tag the information in the footnotes to their financial statements, tends to be significantly more complex than the first. "The face of the financial statements is pretty standardized, which simplifies tagging," says Campbell Pryde, president and chief executive officer with XBRL U.S., a national consortium for XML business reporting standards. "In the footnotes, a lot of information is customized and requires additional work."

As a result, even companies that easily navigate the initial XBRL reporting requirements may find the job gets more difficult. "You may get through this summer," says David Price, chief financial officer with EDGAR Online, a provider of XBRL software, which has been filing in XBRL for three years. "But, next summer could be four to five times as difficult," Price adds, referring to the fact that this is when the latest group of filers will report footnoted information in XBRL.

Price estimates that were set up at his firm for the initial phase took about 40 hours, in addition to the time spent by the an outsource team. In contrast, the set-up for filing the footnotes took a number of weeks. The process now is embedded within the close process and takes about 30 to 40 hours, along with the time spent by the outsource team.

Even as companies work hard to comply with XBRL requirements, it's not clear how many investors are using it. According to the FEI's letter, of the companies that tracked it, none saw more than a slight uptick -- between three and 20 hits per quarter -- in web use after reporting in XBRL.