  We propose a continuous-variable measurement-device-independent quantum key
distribution (CV-MDI QKD) protocol, in which detection is conducted by an
untrusted third party. Our protocol can defend all detector side channels,
which seriously threaten the security of a practical CV QKD system. Its
security analysis against arbitrary collective attacks is derived based on the
fact that the entanglement-based scheme of CV-MDI QKD is equivalent to the
conventional CV QKD with coherent states and heterodyne detection. We find that
the maximal total transmission distance is achieved by setting the untrusted
third party close to one of the legitimate users. Furthermore, an alternate
detection scheme, a special application of CV-MDI QKD, is proposed to enhance
the security of the standard CV QKD system.
