  Steering is a manifestation of quantum correlations that embodies the
Einstein-Podolsky-Rosen (EPR) paradox. While there have been recent attempts to
quantify steering, continuous variable systems remained elusive. We introduce a
steering measure for two-mode continuous variable systems that is valid for
arbitrary states. The measure is based on the violation of an optimized
variance test for the EPR paradox, and admits a computable and experimentally
friendly lower bound only depending on the second moments of the state, which
reduces to a recently proposed quantifier of steerability by Gaussian
measurements. We further show that Gaussian states are extremal with respect to
our measure, minimizing it among all continuous variable states with fixed
second moments. As a byproduct of our analysis, we generalize and relate
well-known EPR-steering criteria. Finally an operational interpretation is
provided, as the proposed measure is shown to quantify the optimal guaranteed
key rate in semi-device independent quantum key distribution.
