Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† ĭdĕa, ae, f., = ιδέα, a (Platonic) idea, archetype, Sen. Ep. 58 med. (in Cic. Or. 3, 10; id. Ac. 1, 8, 30; id. Tusc. 1, 24, 58, as Greek).