In the discussion of the relationship between FHIR Documents and CDA documents, it is stated that:

Composition resources are wrapped into Document structures, for exchange of the whole package (the composition and its parts), and this wrapped, sealed entity is equivalent to a CDA document, where the Bundle.id is equivalent in function to ClinicalDocument.id (but it is not identical when interconverting, since it's a transform between them).

I think that it is Bundle.identifier that is equivalent to ClinicalDocument.id (since both are business identifiers).