We report a measurement of the cosmic ray energy spectrum based on a large amount of data collected by the Pierre Auger Observatory. This measurement combines data from the fluorescence (FD) and surface (SD) detectors of the Observatory and does not rely on detailed numerical simulation or any assumption about the chemical composition. The energy calibration of the observables, which exploits the correlation of surface detector data with fluorescence measurements in hybrid events, is presented in detail. Besides presenting statistical uncertainties, we address the impact of systematic uncertainties. We also summarize the combined energy spectrum obtained when hybrid data are used to extend the spectrum to lower energies.