We investigate a turbo soft detector based on the expectation propagation (EP) algorithm for large-scale multiple-input multiple-output (MIMO) systems. Optimal detection in MIMO systems becomes computationally unfeasible for high-order modulations and/or large number of antennas and approximations are used instead. We propose a new EP-based detector with enhanced estimation in terms of complexity and performance. Specifically, we improve the convergence of the self-iterated EP stage by re-defining the priors to better characterize the information returned by the decoder once the turbo procedure starts. We also review the EP parameters to avoid instabilities when using high-order modulations and to reduce the computational complexity. Simulation results illustrate the robustness and enhanced performance of this novel detector in comparison to previous approaches found in the literature.