be somebody's department

be (not) (one's) department

To be one's area of expertise or responsibility. I can't approve expense reports—that's not my department. Car repair is my dad's department, so he should be able to help you out.
See also: department

be somebody’s department

(spoken) be something that somebody is responsible for or knows a lot about: Don’t ask me about it — that’s Helen’s department, not mine.
See also: department