He's been mainstream forever. He is one of the most covered artists of all time. Everyone knows who he is.
EDIT:
Over two thousand renditions of Cohen's songs have been recorded. He has been inducted into both the Canadian Music Hall of Fame and the Canadian Songwriters Hall of Fame and is also a Companion of the Order of Canada, the nation's highest civilian honour. While giving the speech at his induction into the American Rock and Roll Hall of Fame on March 10, 2008, Lou Reed described Cohen as belonging to the "highest and most influential echelon of songwriters.
The dude's huge!