Kaplansky's idempotent conjecture for torsionfree groups

Let $R$ be a field and let $G$ be a group. Kaplansky's idempotent conjecture. If $G$ is torsionfree, then the group ring $RG$ has only the trivial idempotents, namely $0$ and $1$. The source presents this as a conjectural consequence related to the projective class group conjectures; no resolution status is supplied.