Idaho Code § 55-310: Monuments and fences.

Coterminous owners are mutually bound equally to maintain:

1. The boundaries and monuments between them.

2. The fences between them, unless one of them chooses to let his land lie without fencing, in which case, if he afterward incloses it, he must refund to the other the just proportion of the value, at that time, of any division fence made by the latter.