  The {\gamma}-ray sky can be decomposed into individually detected sources,
diffuse emission attributed to the interactions of Galactic cosmic rays with
gas and radiation fields, and a residual all-sky emission component commonly
called the isotropic diffuse {\gamma}-ray background (IGRB). The IGRB comprises
all extragalactic emissions too faint or too diffuse to be resolved in a given
survey, as well as any residual Galactic foregrounds that are approximately
isotropic. The first IGRB measurement with the Large Area Telescope (LAT) on
board the Fermi Gamma-ray Space Telescope (Fermi) used 10 months of sky-survey
data and considered an energy range between 200 MeV and 100 GeV. Improvements
in event selection and characterization of cosmic-ray backgrounds, better
understanding of the diffuse Galactic emission, and a longer data accumulation
of 50 months, allow for a refinement and extension of the IGRB measurement with
the LAT, now covering the energy range from 100 MeV to 820 GeV. The IGRB
spectrum shows a significant high-energy cutoff feature, and can be well
described over nearly four decades in energy by a power law with exponential
cutoff having a spectral index of $2.32\pm0.02$ and a break energy of
$(279\pm52)$ GeV using our baseline diffuse Galactic emission model. The total
intensity attributed to the IGRB is $(7.2\pm0.6) \times 10^{-6}$ cm$^{-2}$
s$^{-1}$ sr$^{-1}$ above 100 MeV, with an additional $+15$%/$-30$% systematic
uncertainty due to the Galactic diffuse foregrounds.
