Nobody has an insurance.
Taking liability insurance as an example, they either have:
- liability insurance (i.e. used as an uncountable noun), or
- a liability insurance policy (i.e. a specific instance of the paperwork)
They don’t have “an insurance” in the financial sense, at any rate. Keeping a key under the flowerpot next to your door can be an insurance against ever getting locked out, or a pint of milk in the freezer can be an insurance against running out. But even there, many people would drop the definite article.
Prevalence: low. Just a couple of dozen cases. But that’s only because I don’t do much financial work. It’s very common if the term “insurance” occurs at all.
Frequency: common. Usually multiple times in any document where there are any instances.
Native: not really. Although I’ve seen it in American English, I wouldn’t say it’s the norm there either.