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
Farlex Dictionary of Idioms.

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
Farlex Partner Idioms Dictionary
See also:
  • be (not) (one's) department
  • department
  • not (one's) department
  • not your department
  • at the expense of
  • at the expense of somebody/something
  • be ahead of the pack
  • be on (one's) say-so
  • chime
  • chime in