environment. Numerical simulation and low temperature measurements demonstrate how this parasitic loss can be reduced, establishing a Lorentzian lineshape in the resonator frequency response and yielding a loaded Q-factor of 2.4 x 10^5 for niobium devices on sapphire substrates. In addition, we report the dependence of the Q and resonance frequency shift Delta f_0 with input power and temperature in the limit where loss from two-level systems in the dielectric dominate.