A TRACON Controller reported they descended an aircraft below the Minimum Vectoring Altitude.

Narrative

Aircraft X was on a vector for the visual approach Runway XXR he was about 18nm southwest of ZZZ at 5000 ft. He was in a 3;700 ft. MVA when I descended him to 3;000 ft. He was about 3 miles away from a 2800 ft. MVA. That is why he was given the descent to 3000 ft. In the area where I descended him there are no obstructions. The closest obstruction to him was about 8 miles north of his position. His course was diverging away from the obstruction that is 2;400 ft. and was never going to get close to the obstruction. When I descended the aircraft I thought it was going to be clean because there are no obstructions in that area. I was later informed about the MVA bust by the ATM. I have re watched the replay and still think that it was a clean and safe operation. I think there needs to be some clarification on the correct way to descend an aircraft below the MVA when clear of all obstructions. The ATM is under the assumption that no matter where they are at or where the obstruction is you can not descend them before the depicted MVA.