David Letterman through the years

David Letterman attends the American Museum Of Natural History's 2017 Museum Gala on Nov. 30, 2017.

David Letterman attends a game between the St. Louis Cardinals and Cincinnati Reds at Great American Ball Park in Ohio on Aug. 6, 2017.

David Letterman speaks at the 32nd Annual Rock and Roll Hall Of Fame induction ceremony at Barclays Center in Brooklyn on April 7, 2017.

David Letterman speaks at The New Yorker Festival 2016 in New York on Oct. 7, 2016.

David Letterman and his wife, Regina Lasko, arrive for a state dinner for Nordic leaders at the White House on May 13, 2016.

David Letterman and comedian Jon Stewart kick off the fifth anniversary of Joining Forces, an organization for service members, and the 75th anniversary of the United Service Organizations at Andrews Air Force Base in Maryland on May 5, 2016.

Tom Brokaw, right, interviewed David Letterman in May 2016 for a segment on NBC News' "Dateline: On Assignment."

David Letterman during a morning workout on St. Barts in the Caribbean on March 22, 2016.

David Letterman speaks at Ball State University in Indiana, his alma mater, on Nov. 30, 2015.

David Letterman hosts his final broadcast of the "Late Show with David Letterman," at the Ed Sullivan Theater in New York on Wednesday, May 20, 2015.

Bill Murray emerged from a cake to give CBS' "Late Show" host David Letterman a good-bye embrace during a taping of the final show, which aired May 20, 2015.

George Clooney handcuffs himself to David Letterman on the set of the "Late Show with David Letterman" on May 14, 2015.

Howard Stern and Don Rickles say farewell to CBS "Late Show" host David Letterman on May 11, 2015.

President Barack Obama talks on the set of "Late Show With David Letterman" in Manhattan on Sept. 18, 2012.

Lady Gaga visited "Late Show with David Letterman" on May 23, 2011.

David Letterman and former race driver Bobby Rahal talk before the 93rd annual Indianapolis 500 auto race at the Indianapolis Motor Speedway on May 24, 2009.

Actor Joaquin Phoenix waves to the audience during his interview on the "Late Show with David Letterman" on Feb. 11, 2008.

New York City Mayor Rudolph Giuliani visits "Late Show with David Letterman" on Dec. 27, 2001.

David Letterman gets a champagne bath from members of the World Championship New York Yankees during a taping of the show on Oct. 28, 1996.

Actress Drew Barrymore turns to the audience after flashing host David Letterman during the April 12, 1995, taping of "Late Show with David Letterman" in Manhattan.

David Letterman announces his new contract with CBS television for "Late Show with David Letterman" in Manhattan on Jan. 15, 1993.