Metaphor and metonymy are likely the most common forms of non-literal language. As metaphor and metonymy differ conceptually and in how easy they are to comprehend, it seems likely that they also differ in their degree of non-literalness. They frequently occur in idioms which are foremost non-literal, fixed expressions. Given that non-literalness seems to be the defining criterion of what constitutes an idiom, it is striking that no study so far has focused specifically on differing non-literalness in idioms. It is unclear whether and how metaphoric and metonymic structures and their properties are perceived in idioms, given that the comprehension of idioms is driven by a number of other properties that are connected. This study divides idioms according to their metonymic or metaphoric structure and lets participants rate their non-literalness, familiarity, and transparency. It focuses on non-literalness as key property, finds it strongly connected to transparency, and to be the one key factor in predicting idiom type. Specifically, it reveals that metonymies are generally perceived as rather or even extremely literal, while metaphors are generally perceived as highly non-literal.