Sounds like you'll be wanting at least a fireproof safe, to keep your documents safe from a fire. If you also want to keep those documents safe from a criminal you should step your search for a safe up to a Burglary/Fire safe. The burglary fire safes I have seen are all just under a UL TL-15 listing & they are much heavier than a standard type safe.
For example, I have two of Amsec's BF Series gun safes, the smaller one weights 885 pounds & the bigger one weighs 1850. With that kind of weight, I don't bother fastening them to anything-- I'm pretty sure they won't be going anywhere
These are the smaller ( & lighter BF safes)
https://americansecuritysafes.com/produ ... -security/The above have a one-hour fire rating but the heavier BF series have a two hour fire rating
https://agunsafe.com/Amsec_Gun_Safes.html