I like having levels of security, so I have a safety deposit box (with most of the "original" originals- BC's, marriage stuff, passports for periods of non travel savings bonds. We are fortunate enough to be able to share with 2 adult children and spouses, so it does tend to be full at most times. I have scanned nearly every document in it so that serves as my emergency backup, and the scans are backed up on 3 external drives that I rotate around to the kids houses.
On premise I have a safe (rarely locked), a fire resistant cabinet and a locked down place for firearms (want to upgrade to a proper gun safe at some point). I keep the insurance papers, pension info and tax returns in the file cabinet, passports near travel times in the safes and firearms/ ammo all hang out in there designated spaces.
I have found that the very nature of organizing everything has made "missing items" a thing of the past. My daughter recently needed to provide her measles vaccination from 1990 to her daycare center (she is the Director). Dear old dad, who has been roundly mocked for his "scanning habit" was able to produce said document in under 10 minutes... big win for Dad here!
From a larger perspective, having come professionally from the Business Continuity space, I steer interested people to this deep, deep website written by a guy who lost his home in Katrina. I have not put into place nearly 80% of what he suggests, but what I have put in place to manage critical documents came largely from his perspective. His "experiences" have been actually tested:
http://www.theplacewithnoname.com/blogs/klessons/index.html