Cosmopolitan has partnered with Marriott International. Cosmo will become part of Marriott's Autograph Collection. These hotels are not owned or operated by Marriott but provide the hotel access to Marriott's customers through their booking engine and rewards program. In turn Marriott will get a cut of the hotel revenues, they will not however get any of the gambling revenue at Cosmo. Most of the Autograph Collection hotels are small, boutique places - Cosmopolitan will be the largest hotel in the Autograph Collection.
This certainly seems like a mutually beneficial partnership. Marriott gets access to a prime hotel on the strip, while Cosmo gets access to Marriott's massive customer base and booking engine.
Source: AP.