The OED definition of idiolect (see below), would suggest it is something personal. but I have seen it used, including on this site as a word which describes the common idiomatic content of a community. It is used like dialect but referring specifically to idioms. This post for example talks about the Indian idiolect.
So does the OED reflect the way the word is used or not?
The linguistic system of one person, differing in some details from that of all other speakers of the same dialect or language.
1948 B. Bloch in Language 24 7 The totality of the possible utterances of one speaker at one time in using a language to interact with one other speaker is an idiolect.
1964 M. A. K. Halliday et al. in J. A. Fishman Readings Sociol. of Lang. (1968) 158 A person's idiolect may be identified, through the lens of the various registers, by its grammatical and lexical characteristics.
1975 R. L. Williams Ebonics p. vi, Ebonics..includes the various idioms, patois, argots, ideolects, and social dialects of black people.
2001 S. S. Mufwene Ecol. Lang. Evol. viii. 193 Every new speaker replicates their target communal language imperfectly, starting with the trivial fact that they couldn't possibly replicate all the idiolects of which it is an ensemble and no idiolect replicates another.