It looks like Hymenocallis 'Tropical Giant', a large one with very broad leaves. Do three of the six tepals recurve down more than the others? It has a 'sister' which doesn't have this peculiarity.
http://www.pacificbulbsociety....
I managed to get one of these but it's barely growing even keeping it in a greenhouse over summer! It's one of the more desirable hard to get ones.
