The Securities and Exchange Commission (SEC) has adopted the eXtensible Business Reporting Language (XBRL) in a multi-year program to enhance the functionality of the Commission's EDGAR database. Filers tag their financial statements with elements from a taxonomy that defines the reporting concepts so that the XBRL files can be understood by information consumers. The U.S. GAAP taxonomy was designed to represent common reporting practices and support the disclosure requirements of U.S. GAAP. If taxonomy elements for each disclosure concept are not present, the filer creates an extension element. Extensions, when used appropriately, provide decision-relevant information. When used inappropriately, particularly when a semantically equivalent element already exists in the foundation taxonomy, extensions add no information content. This research analyzes extensions made in a subset of XBRL filings made to the SEC between April 2009 and June 2010. Forty percent of these extensions were unnecessary, as semantically equivalent elements were already in the U.S. GAAP taxonomy. Extensions that aggregated or disaggregated existing elements comprised 21 percent of the extensions. New concepts accounted for 30 percent of the extensions, although many were variants of existing elements, rather than significantly new concepts.