David Letterman's long goodbye begins. On April 3, the man who has been an institution of late-night network TV for more than three decades announced he would retire in 2015. On Late Show, R.E.M.'s Mike Mills — who broke the news — and Peter Buck helped back urbane singer-songwriter Joseph Arthur for an elegiac cover of Lou Reed's "Walk on the Wild Side." Watch that above, and check out Letterman's retirement announcement below. No Garth Brooks for this guy, eh, Paul?