  We report a new measurement of electron antineutrino disappearance using the
fully-constructed Daya Bay Reactor Neutrino Experiment. The final two of eight
antineutrino detectors were installed in the summer of 2012. Including the 404
days of data collected from October 2012 to November 2013 resulted in a total
exposure of 6.9$\times$10$^5$ GW$_{\rm th}$-ton-days, a 3.6 times increase over
our previous results. Improvements in energy calibration limited variations
between detectors to 0.2%. Removal of six $^{241}$Am-$^{13}$C radioactive
calibration sources reduced the background by a factor of two for the detectors
in the experimental hall furthest from the reactors. Direct prediction of the
antineutrino signal in the far detectors based on the measurements in the near
detectors explicitly minimized the dependence of the measurement on models of
reactor antineutrino emission. The uncertainties in our estimates of
$\sin^{2}2\theta_{13}$ and $|\Delta m^2_{ee}|$ were halved as a result of these
improvements. Analysis of the relative antineutrino rates and energy spectra
between detectors gave $\sin^{2}2\theta_{13} = 0.084\pm0.005$ and $|\Delta
m^{2}_{ee}|= (2.42\pm0.11) \times 10^{-3}$ eV$^2$ in the three-neutrino
framework.
