Though Seacrest -- known for quarreling with Cowell on the show -- sent him a congratulatory message, Cowell tells Extra (in his first interview since his exit announcement): "I heard a lot of popping champagne bottles. There will be a fireworks display outside his house tonight."
Cowell said he thought about juggling both Idol and the American version of his British hit music show, The X Factor, but decided it would be best to "concentrate on one main show." The X Factor will air on Fox in 2011.
Leaving Idol - which he helped launch in 2002 - is "emotional," he tells Extra. "It did feel a little bit emotional when I [made the announcement] because then it's real... and then you remember all the good times you had."
On her talk show Tuesday, new judge Ellen DeGeneres says she is "trying not to take it personally." Joking, she added, "But seriously, I am going to be very, very sad to see him go, because I think he's made the show what it is. He's a huge part of that show but he wanted a change. I wish him all the luck in the world hosting The Tonight Show."
Judge Kara DioGuardi says she was "the last person to know" about Cowell's exit. "He is so important to the show," she said on 104.3 MY FM's Valentine in the Morning show in Los Angeles Monday. "He will be missed."