As someone whose mother retired from a state Atty. Generals office and whose wife is a Norwegian legal assistant, I can tell you forvaring is very foreign to the American reference frame. Sufficiently so that I have been discussing it with my wife since before Breiviks murders in Oslo and on Utøya, and I still do not have a firm grasp of it. Breivik will not simply be released in 21 years though; the forvaring law was created about a decade ago to ensure that sort of thing does not happen, and Breivik will almost certainly be in prison for life.