Google, AFAIK the only company with cars that are actually autonomous, has US$98 Billion in cash.
It'd have to be a hell of an accident to put a dent in that.
All unlimited liability insurance companies (e.g. motor insurers in the UK) have reinsurance to take the hit on claims over a certain level - e.g. 100k, 1m etc.
For extreme black swan risks, this is how you prevent the insurance company just going bankrupt.
Reinsurers themselves then also have their own reinsurance, and so on. The interesting thing is that you then have to keep track of the chain of reinsurers to make sure they don't turn out to be insuring themselves in a big loop. A "retrocession spiral" could take out many of the companies involved at the same time, e.g. the LMX spiral.
You'd need an immensely rich or influential opponent to decide they wanted to march through hell in order to hold Google's feet to the fire. It'd have to be something deeply personal and they probably have things structured to limit any potential liability to a couple hundred million. They'll never be held to account for anything that goes seriously wrong.
Getting richer has never made someone LESS likely to be assured.