  Einstein-Podolsky-Rosen steering incarnates a useful nonclassical correlation
which sits between entanglement and Bell nonlocality. While a number of
qualitative steering criteria exist, very little has been achieved for what
concerns quantifying steerability. We introduce a computable measure of
steering for arbitrary bipartite Gaussian states of continuous variable
systems. For two-mode Gaussian states, the measure reduces to a form of
coherent information, which is proven never to exceed entanglement, and to
reduce to it on pure states. We provide an operational connection between our
measure and the key rate in one-sided device-independent quantum key
distribution. We further prove that Peres' conjecture holds in its stronger
form within the fully Gaussian regime: namely, steering bound entangled
Gaussian states by Gaussian measurements is impossible.
