Full Idea
To say that Leibniz is an idealist is to say that simple substances, the basic building-blocks of the universe, are all mental or at least quasi-mental in nature
Clarification
'Quasi-' means somewhat
Gist of Idea
Leibniz is an idealist insofar as the basic components of his universe are all mental
Source
report of Gottfried Leibniz (works [1690]) by Nicholas Jolley - Leibniz Ch.3
Book Reference
Jolley,Nicholas: 'Leibniz' [Routledge 2005], p.88
A Reaction
This is a bit different from the Berkelian type of idealism, which says that reality consists entirely of events within thinking minds. Is a monad the thinker or the thought?