Recently, a variety of low complexity soft-input soft-output detection algorithms have been introduced for iterative detection and decoding (IDD) systems. However, it is still challenging to implement soft-input soft-output detector detector at feasible complexity for massive MIMO systems due to the heavy burden in computational complexity. In this paper, we present a novel soft-input soft-output detector for massive MIMO systems that offers substantial complexity reduction over the existing detectors performance close to the full dimensional symbol detector. The proposed soft-input soft-output successive group (SSG) detector detects a subset of symbols successively with an aid of the deliberately designed preprocessing to suppress the inter-group interference. Simulation results performed on 8×8 MIMO flat fading channels demonstrate that the proposed SSG detector achieves significant complexity reduction over the conventional approaches with negligible performance loss.