Yeah, they're not really an insurance company, but probably 2000+ companies with billions of dollars of risk.
They can insure your 10-million-dollar mansion on the San Andreas fault ---- for a mere $500K a year or somesuch --- because they similarly insure 10-million-dollar mansions on the flood plane of the Mississippi --- for the same price --- on the theory that not everything falls apart all over the world at the same time.
My rigs (oil rigs) are casualty-insured by probably 10 different companies at the same time under the same idea.
Big risks. Big costs.
First of all, ALL the wiring, heating, cooling, and cabling are OUTSIDE the main structure. The insides are business ONLY. (you step "outside" the building to go to the bathroom).
Some guy at a desk says "I have 500 million dollars of risk on some condos here in Myrtle Beach SC, revenue from the policies at x amount of dollars, loss ration 82.4% for last three years..." and some giant consortium of Arabs, or some big financial somebody says "I will take that!" and you have someone "buying" the cash by assuming the risk and the deal is done! It is very heady.
As an aside, there are about 11 big conglomerates who "underwrite" the big big big risks at Lloyds. These are the people who stand BEHIND the Travelers, and the Hartfords, and the Chubbs, and all. Eight of those were functionally bankrupt after 9/11, in that they did not have the cash reserves mandated to continue to underwrite business. We don't know how close the entire world insurance market came to collapsing. There was an agreement to allow these guys to continue to function with reduced margin requirements, in the hopes that they could build up equity and continue to function, so as not to roil the mkts more than they already were.
It is really pretty interesting to look into.