I think the peculiarity of our discussion of Kant (or our inevitable reference to Kant regardless of the topic) is that Kant did not think of himself as an idealist – idealism for him is a position that declares that
…the existence of objects in space outside us to be either merely doubtful and indemonstrable, or else false and impossible; the former is the problematic idealism of Descartes, who declares only one empirical assertion (assertio), namely that I am, to be indubitable; the latter is the dogmatic idealism of Berkeley, who declares space, together with all the things to which it is attached as an inseparable condition, to be something that is impossible in itself, and who therefore also declares things in space to be merely imaginary. [B274]
Interestingly enough, Kant is not spending much time on dogmatic idealism of Berkeley, only stating that
Dogmatic idealism is unavoidable if one regards space as a property that is to pertain to the things in themselves; for then it, along with everything for which it serves as a condition, is a non-entity. [B274]
Space that is a property of things in themselves is absolute space. Kant argues that absolute space as an existing thing would be a non-entity, a thing whose concept is self-contradictory: according to Kant’s neat table of “nothing” [A292/B349], absolute space would be nihil negativum, “empty object without concept” – “even its concept cancels itself out” – Kant already argued in Transcendental Aesthetics that space cannot be an entity, a thing like other things that are found in space. His argument here is close to Leibniz’s argument against Clarke/Newton and the notion of absolute space – an idea of space as a container for things, as a substance is contradictory.
What is interesting in the above citation however is that Kant sees the very move that one would think a “realist” would make – ascribe space and time to objects, not to the perceiving subject – as that which creates in turn the worst kind of idealism, a dogmatic idealism that declares space and everything in it to be an illusion.