I'd do it by converting the OpenXML docx to XHTML via XSLT.
My XSLT would ask "does the content control have a tag containing any special instruction?"
Such an instruction might be: output this content control as element us-gaap:DebtDisclosureTextBlock
In which case whatever the content control contained would be wrapped in that element.
If you take this approach, you need:
a way of doing the docx to XHTML which acts on the instructions in your content control tag
a way of adding the content control tag in the first place (you can do this via the Word Ribbon - Developer tab, but you might want to give your users a way to choose the appropriate XBRL element)