In this paper, we propose to decompose the received signal into its projection on the channel vector and the corresponding perpendicular in the spatial modulation (SM) detection. Based on this decomposition, the optimal joint antenna and symbol detection can be done separately with reduced complexity and no performance loss. Moreover, the constellation symmetry and a lower bound approximation are employed in the antenna detection to reduce its complexity further. Simulation results are provided to demonstrate the performance of the proposed detectors.