  We report the observation and physical characterization of the possible dwarf
planet \UZ\ ("DeeDee"), a dynamically detached trans-Neptunian object
discovered at 92 AU. This object is currently the second-most distant known
trans-Neptunian object with reported orbital elements, surpassed in distance
only by the dwarf planet Eris. The object was discovered with an $r$-band
magnitude of 23.0 in data collected by the Dark Energy Survey between 2014 and
2016. Its 1140-year orbit has $(a,e,i) = (109~\mathrm{AU}, 0.65,
26.8^{\circ})$. It will reach its perihelion distance of 38 AU in the year
2142. Integrations of its orbit show it to be dynamically stable on Gyr
timescales, with only weak interactions with Neptune. We have performed
followup observations with ALMA, using 3 hours of on-source integration time to
measure the object's thermal emission in the Rayleigh-Jeans tail. The signal is
detected at 7$\sigma$ significance, from which we determine a $V$-band albedo
of $13.1^{+3.3}_{-2.4}\mathrm{(stat)}^{+2.0}_{-1.4}\mathrm{(sys)}$ percent and
a diameter of $635^{+57}_{-61}\mathrm{(stat)}^{+32}_{-39}\mathrm{(sys)}$~km,
assuming a spherical body with uniform surface properties.
