There's this one called simply "Giant Lily"
Giant Lily (Cardiocrinum giganteum) but yours don't have the red splash inside the trumpets.
There's also a Giant Himalayan Lily, and it has an entry in our database but no picture or info. I know I saw it in a catalog not too long ago but this is the only entry I can find with a picture. This one says it can get to 10ft. so, might be the one. Check the rosette of leaves at the base, that seems to be a clue to the ID as at least a Cardiocrinum.
http://www.burpee.com/perennia...
Welcome to ATP and hope this helps you out, it's really pretty whatever it is!