This paper proposes a syntactic account on interpretations of idioms, which vary depending on voice alternations in Korean. There exist asymmetries on whether an idiom maintains its idiomatic interpretation despite its voice alternation. This paper shows that the principle of idiomatic interpretations (Marantz 2008, Bruening 2010) directly accounts for the asymmetries of idiomatic interpretations, coupled with the syntactic structures of two types of passives and causatives proposed in this paper. The discussions in this paper support the theory of Distributed Morphology, which holds that idiomatic interpretations are post-syntactically added from the Encyclopedia.