We present distributions of shower depth of maximum ($X_{\mathrm{max}}$) and their interpretation in terms of the cosmic ray mass composition. The measurements of $X_{\mathrm{max}}$ are based on data from the fluorescence telescopes of the Pierre Auger Observatory. Due to the extension of the field of view with the High Elevation Auger Telescopes, a lower energy threshold of $10^{17.2}\,$eV can be reached. At the highest energies we have increased the available statistics by including three more years of data in the analysis compared to our previous publications. We present estimates of the first two moments of the $X_{\mathrm{max}}$ distribution and of the composition fractions over a large energy range, from $10^{17.2}\,$eV to about $10^{19.6}\,$eV. The composition fractions are estimated by fitting the $X_{\mathrm{max}}$ distributions with four elemental groups represented by p, He, N and Fe and using post-LHC hadronic models.