LOS ANGELES (Reuters) - Rapper Nicki Minaj and country singer Keith Urban have signed up as judges for TV singing contest "American Idol," Fox television said on Sunday. They will join Grammy-winning artist Mariah Carey for the 12th season of the show when it returns in January 2013. It lost some 20 percent of its audience last season. Randy Jackson, the only remaining member of the original panel, will remain with the show. The new judging line-up follows the departures earlier this year of Aerosmith frontman Steven Tyler and singer-actress Jennifer Lopez after two years on the show. ...