Longtime late-night host David Letterman has been honored with the nation's top prize for comedy. The Kennedy Center announced Tuesday that the 70-year-old Letterman is this year's recipient of the Mark Twain Prize for American Humor. He is the 20th humorist to receive the annual prize, which began in 1998. Last fall, he delivered a warm tribute on stage at the Kennedy Center as his frequent guest, Bill Murray, accepted the award. Letterman hosted more than 6,000 episodes of late-night television, starting in 1982 with NBC's "Late Night with David Letterman." He moved to CBS in 1993 and hosted "The Late Show" until his retirement two years ago. Letterman's irascible, independent streak inspired fierce loyalty from fans and critics.