Neutrino telescopes provide some of the best sensitivities for heavy decaying dark matter. With IceCube's observation of high-energy astrophysical neutrinos, interest in scenarios that could explain their origin as a result of the decay of long-lived heavy dark matter particles has risen. We present two dedicated experimental analyses to test this scenario of decaying dark matter with masses above 10 TeV. One analysis uses 6 years of IceCube data focusing on muon-neutrino tracks from the northern hemisphere and one analysis uses 2 years of cascade data from the full sky. The following contributions to the neutrino flux are considered: Atmospheric neutrinos, a diffuse astrophysical flux following a power-law spectrum and a potential flux of neutrinos produced in dark matter decays. The latter can be distinguished by its distinctive features in the energy spectrum (cut-off at half the mass of the DM-particle) and asymmetry of the arrival directions due to the DM halo of our galaxy. We present best-fit results and deduce lower lifetime limits on the order of $10^{28}$ sec for dark matter masses above 10 TeV. \\