  Debris disks are dusty, optically thin structures around main sequence stars.
HD 106906AB is a short-period stellar binary, host to a wide separation planet,
HD 106906b, and a debris disk. Only a few known systems include a debris disk
and a directly imaged planet, and HD 106906 is the only one in which the planet
is exterior to the disk. The debris disk is edge-on and highly asymmetric in
scattered light. Here we resolve the disk structure at a resolution of 0.38"
(39 au) with the Atacama Large Millimeter/submillimeter Array (ALMA) at a
wavelength of 1.3 mm. We model the disk with both a narrow and broad ring of
material, and find that a radially broad, axisymmetric disk between radii of
$\sim$50$-$100 au is able to capture the structure of the observations without
evidence of any asymmetry or eccentricity, other than a tentative stellocentric
offset. We place stringent upper limits on both the gas and dust content of a
putative circumplanetary disk. We interpret the ALMA data in concert with
scattered light observations of the inner ring and astrometric constraints on
the planet's orbit, and find that the observations are consistent with a
large-separation, low-eccentricity orbit for the planet. A dynamical analysis
indicates that the central binary can efficiently stabilize planetesimal orbits
interior to $\sim$100 au, which relaxes the constraints on eccentricity and
semimajor axis somewhat. The observational constraints are consistent with in
situ formation via gravitational instability, but cannot rule out a scattering
event as the origin for HD 106906b's current orbit.
