This paper examines the effect of market participants’ information processing costs on firms’ disclosure choice. Using the recent eXtensible Business Reporting Language (XBRL) regulation as an exogenous shock to these information processing costs, but not to firms’ disclosure requirements, I find that firms increase their quantitative footnote disclosures upon implementation of XBRL detailed tagging requirements designed to reduce information users’ processing costs. These results hold in a difference-in-difference design using matched non-adopting firms as controls, as well as two additional identification strategies. To provide further evidence that the disclosure increase is prompted by reduced information processing costs, I examine cross-sectional settings where the importance of market participants’ processing costs is likely to vary, showing that the disclosure increase is often greater for firms where detailed information is more pertinent and smaller for firms with more visibility. In addition, examination of the disclosure increase by type of footnote suggests that both regulatory and non-regulatory market participants play a role in monitoring firm disclosures. Overall, these findings suggest that the processing costs of market participants can be significant enough to impact firms’ disclosure decisions.