In general a child’s home state is where the child lived for at least six consecutive months immediately before commencement of a child custody proceeding, although there are exceptions and clarifications. The Uniform Child Custody and Jurisdiction Enforcement Act (UCCJEA) was passed by all 50 states as an attempt to reduce multiple states making a determination of custody that conflict. Each state passed a slightly different version of the UCCJEA and can be very confusing and complicated.
Determining which state to file a custody action is very complicated and you should contact an attorney immediately, because it is extremely time sensitive and unforgiving in some senses. You might find yourself fighting custody in a far away state which could have been avoided if you had filed first or spoke to an attorney first.
Just because you have a child support order from another state that may not prevent you from filing for custody in Idaho. That child support order may not be a “child custody determination,” according to the UCCJEA.
Idaho Code 32-11-102
(c) "Child custody determination" means a judgment, decree, or other order of a court providing for the legal custody, physical custody or visitation with respect to a child. The term includes a permanent, temporary, initial and modification order. The term does not include an order relating to child support or other monetary obligation of an individual.
Idaho Code 32-11-201
Idaho has jurisdiction if “This state is the home state of the child on the date of the commencement of the proceeding”
Idaho Code 32-11-102(g)
(g) "Home state" means the state in which a child lived with a parent or a person acting as a parent for at least six (6) consecutive months immediately before the commencement of a child custody proceeding. In the case of a child less than six (6) months of age, the term means the state in which the child lived from birth with any of the persons mentioned. A period of temporary absence of any of the mentioned persons is part of the period.
(Last viewed 12/17/2015, http://legislature.idaho.gov/idstat/Title32/T32CH11PT1.htm)