According to Guns, Germs, and Steel, what was the Mongols' political structure?

Diamond mentions two different political structures that
the Mongols had.  These structures changed over time.  At first, the Mongols' society
was a chiefdom.  Later, as the Mongols were able to conquer an empire, their society
turned into a state.

At the end of Chapter 13, Diamond
states that the Mongols who conquered China were a chiefdom.  At this time, they did
have a centralized government, but it was less organized.  It was not the sort of
government that the Mongols later had when they had conquered China.  At that point,
they had a highly bureaucratized government wtih laws and judges, not a more informal
system where issues of justice were decided more
arbitrarily. 

So, the Mongols had a chiefdom but then
developed a state as they increased the size of their empire.