A downhole tool includes a streamlined housing sized to freely move through a wellbore formed from a terranean surface to a subterranean formation; an explosive material enclosed within an interior volume of the housing, the explosive material sufficient to cut at least a portion of a tubular positioned in the wellbore upon detonation; a detonator coupled to the explosive material; and a detonator control system communicably coupled to the detonator and configured to activate the detonator with a detonation signal to detonate the explosive material.