  We present 2-9 GHz radio observations of GW170817 covering the period 125-200
days post-merger, taken with the Australia Telescope Compact Array and the Karl
G. Jansky Very Large Array. Our observations demonstrate that the radio
afterglow peaked at $149\pm2$ days post-merger and is now declining in flux
density. We see no evidence for evolution in the radio-only spectral index,
which remains consistent with optically-thin synchrotron emission connecting
the radio, optical, and X-ray regimes. The peak implies a total energy in the
synchrotron-emitting component of a ${\rm few}\times 10^{50}\,$erg. The
temporal decay rate is most consistent with mildly- or non-relativistic
material and we do not see evidence for a very energetic off-axis jet, but we
cannot distinguish between a lower-energy jet and more isotropic emission.
