Not very detailed knowledge I have but knows that following things are performed by UE in RRC IDLE mode.
1. Cell measurement (Measure the signal quality of neighbor cells)
2. Cell search/PLMN search to determine the cells on which it would camp on.
3. Cell selection
4. Cell re-selection
Each point mentioned above is a major topic in itself. For more details , go through 3GPP 36.304 specification.