  If the dark matter is unstable, the decay of these particles throughout the
universe and in the halo of the Milky Way could contribute significantly to the
isotropic gamma-ray background (IGRB) as measured by Fermi. In this article, we
calculate the high-latitude gamma-ray flux resulting from dark matter decay for
a wide range of channels and masses, including all contributions from inverse
Compton scattering and accounting for the production and full evolution of
cosmological electromagnetic cascades. We also make use of recent
multi-wavelength analyses that constrain the astrophysical contributions to the
IGRB, enabling us to more strongly restrict the presence any component arising
from decaying dark matter. Over a wide range of decay channels and masses (from
GeV to EeV and above), we derive stringent lower limits on the dark matter's
lifetime, generally in the range of $\tau \sim (1-5)\times 10^{28}$ s.
