XBRL is a globally used and recognised financial reporting standard, built ontop of XML and various XML technologies. It introduces the .xbrl filename extension, used on XBRL Instance documents. Said documents are essentially just entirely valid and standard XML given special semantics by the XBRL specification, in the same way that XML Schema files (.xsd) are just standard XML given special semantics by the XML Schema Definition specification.

Therefore I propose associating *.xbrl with the pygments.lexers.html.XmlLexer. I will open a pull request once I figure out exactly how it should be done.