It's not there but probably should be. Could it roughly be equivalent to the region of Edom?
There is a good description of the boundaries of Idumea in the Anchor Dictionary:
A territory that during the Second Temple period stretched approximately from the S portion of the Judean hill country to the N part of the Negeb. To the N, the borders ran between Beth-Zur and Alouros (Ḥalḥūl), while the S border reached the height of the Arad-Malatha-Beersheba-Aroer line (cf. the so-called Limes of Herod and the Limes Palastinae; Gichon 1967; 1975). To the E, Idumea bordered the Judean desert or the Dead Sea (and perhaps the N tip of the Wadi Arabah), and its territory reached W into the provinces of the port cities Gaza, Ashkelon, and Ashdod. Among the most important Idumean cities mentioned in the literary texts are Hebron, Marisa (Tell Sandaḥanne), Adora (Dūrā), and Betabris (Bēt Gibrı̄n). Whoever ruled Idumea controlled a share of the profitable E–W trade between the Mediterranean coast and Transjordan as well as the N–S trade routes. They could also protect the hinterland against Nabatean and bedouin raids.