With network diagrams you often find that by determining exactly what you want the structure of the diagram will determine itself. The types of diagrams will generally fall into either the logical or physical categories, and can then be drawn accordingly.
It may be that you would like to know where the network equipment is situated in each building, so a series of diagrams for each building could be drawn showing where each item is located within a comms rack. Alternatively, you may be more interested in the physical connections, in which case you could start with a high level diagram of the 11 buildings, with just the one or two routers in each that handle the inter-building connections shown. You could then have further diagrams for each building to show the internal connections to other routers / switches / critical servers etc. with the diagram constructed to clearly show the interconnections rather than the precise location of the equipment.
A logical diagram may be more concerned with the location of VLANs and associated trunks, IP numbering domains etc. This can be high level, showing which VLANs / IP ranges are used within the various switches, trunks or buildings, or low level where you indicate which VLAN is assigned to which port.
One word of advice - unless your network is incredibly stable, don't go down to too much detail on the diagrams as they will either require a large amount of maintenance to record moves and changes, or go out of date quickly. Show what is required to help you in your job, especially in troubleshooting situations, and no more.
