An American Airlines Boeing 737-800 (N925NN), contacted a Southwest Airlines Boeing 737-700 (N784SW), that was stationary on a deice pad with engines stopped and was being deiced at Detroit Metropolitan Airport (DTW). The American Airlines B737 was taxiing to an adjacent deice pad when it struck the Southwest Airlines B737. N925NN incurred damage to the right winglet, and N784SW sustained substantial damage to its right elevator, elevator tab, stabilizer leading edge, and APU exhaust. There were no injuries on either airplane, and both airplanes taxied back to gate to deplane passengers. According to the Southwest flight crew, N784SW was stationary on the deice pad 3 with the engines shut down and was being deiced. According to the deice ground crew, N784SW and a United airplane on deice pad 4 were not fully pulled into their respective parking spots on the deice pad. N925NN passed behind N784SW and turned to enter to an adjacent deice pad, and its right winglet contacted the right elevator, horizontal stabilizer, and APU of the Southwest B737. Probable Cause: N925NN flight crew’s incorrect evaluation of the clearance between the two aircraft, leading to the inadvertent collision of the winglet with the horizontal stabilizer and elevator of N784SW