Why do first ionisation energies decrease down a group?

Ionisation energy is defined as the energy required to remove an electron from a gaseous atom or ion. As we descend down a periodic group we must think about how the atomic radius. As we descend the group the atomic radius begins to increase. This means that the electrons are increasing in distance from the nucleus of the atom. The protons in the nucleus tend to increase the attraction of the electrons to the nucleus. As such if the electrons are further away from this attraction they are more easily removed from the atom. As such the energy required to remove the electron is less as we descend the group.