IDEA Full Funding Act This bill amends the Individuals with Disabilities Education Act to reauthorize and make appropriations for the grant program to assist states and outlying areas in providing special education and related services to children with disabilities. The bill sets the amount to be appropriated for each fiscal year from FY2018-FY2026 as the greater of: (1) a specified amount, or (2) a specified percentage of an amount determined pursuant to a formula that multiplies the number of children receiving special education services by the average per-pupil expenditure in public elementary and secondary schools. The bill appropriates funds for FY2027 and each subsequent fiscal year equal to the greater of a specified amount or 40% of the amount determined using such formula.