You could have registered the idea with the Writer's Guild by writing up the idea, mailing it in
to them for registration. When they returned it,stamped with the date, you'd then have proof that it was your original concept. Barring that, you and the other 5,000 people who ALSO had the idea have nothing but bragging rights.
By the way, studios and production companies never really look at unsolicited materials for that very reason so chances are, they never even saw your submission. Consider it a case of brilliant minds thinking alike.