Fox isn't saying a word about replacing the judges on American Idol so far, but the Entertainment Chairman said it is no shock to him that Randy Jackson is leaving the show.  Kevin Reilly said Jackson has been a great partner for 12 years and there is no ill will between him and the show.   Reilly also said the first thing they will do with American Idol for the 13th season is to tinker with the show's structure.  He said the format needs to have some fresh twists that will add a jolt to next season.   American Idol's current season wraps-up Thursday night with Kree Harrison in a face-off with Candice Glover.   This season's ratings have been dismal for the show that once ruled the airwaves.