A search for a heavy Higgs boson in the H → WW and H → ZZ decay channels is reported. The search is based upon proton-proton collision data samples corresponding to an integrated luminosity of up to 5.1 fb⁻¹ at (\sqrt{s})=7TeV and up to 19.7fb⁻¹ at (\sqrt{s})=8TeV, recorded by the CMS experiment at the CERN LHC. Several final states of the H → WW and H → ZZ decays are analyzed. The combined upper limit at the 95% confidence level on the product of the cross section and branching fraction exclude a Higgs boson with standard model-like couplings and decays in the range 145 < (m_H) < 1000 GeV. We also interpret the results in the context of an electroweak singlet extension of the standard model.