it will NOT be attached to my bow
Yes, you can use a rangefinder while bow hunting big game in Idaho. It cannot be attached to the bow.
The rule states; No person shall take big game animals with any electronic or tritium-powered device attached to, or incorporated into, an arrow, bolt, crossbow, or bow (except nonmagnifying scopes containing battery powered or tritium lighted reticles may be used
by disabled archery permit holders).