Complex sentence: While in prison, Keil first developed the idea for the publication which was to bring him fame and outlive him by many years: an illustrated magazine that would bring "intellectual exercise and education as entertainment" to the bourgeoisie.
Split into simpler sentences: While in prison, Keil first developed the idea for the publication which was to bring him fame and outlive him by many years. It was to be an illustrated magazine that would bring "intellectual exercise and education as entertainment" to the bourgeoisie.