EMB-175 First Officer reported encountering wake turbulence on descent into IAD 7 miles in trail of a B787.

Narrative

While en route to ZZZ from JFK; the airplane encountered wake turbulence approximately 20 minutes prior to landing (17000 feet). As trained; the autopilot was disconnected and the aircraft was hand flown to return to normal attitude and slowed to 270 KIAS. After ensuring the aircraft was stable; the autopilot was engaged again and the Captain notified ATC about the encounter. ATC then communicated that there was a heavy Aircraft Y only 7 miles ahead of us on the arrival. We continued slowing the aircraft and monitored the flightpath to ensure no similar encounters occurred. The flight continued without anymore disturbances.The cause of this would have been a lack of communication and proactiveness on behalf of ATC. Only after communicating to ATC that we encountered wake turbulence were we made aware of the heavy aircraft that was only 7 miles ahead of us on the same path.To avoid recurrence of such events; ATC should increase the spacing when a smaller aircraft follows a heavy aircraft or at least communicate with pilots that they are following a heavy aircraft. Usually this advisory comes during takeoff and landing but we should have received this advisory en route also.