The XBRL International has issued Master File versus Transaction: Some Ramifications - a.k.a. Defining Defaults without Defiance 1.0  in February 17, 2009.

The purpose of this document is to discuss in particular how some data fields have slightly different meanings when used as “defaults” or describers in master files and used differently in transactional representations.

To define a default value: In a typical system, files that provide “default” values are allowing the entry of a value that will be 1) automatically suggested; 2) used unless a specific value is provided by the user, or 3) used unless overridden in later entries of all sorts. As we will see, some fields in XBRL GL serve both purposes – and sometimes their use as a default may be less than obvious at first glance.