Detectives or any officer above a patrol level cannot "transfer in" to a police department from another city. While this is commonly shown as happening on police shows, in fact the only law enforcement position ever filled laterally by an outsider on occasion is the chief. All others are promoted from within, and must meet police standards for the state in which they work. In the example shown here, a detective working for the Atlanta police department would need to resign, and then be hired by the NYPD, attend training, and become certified for New York. They would then start as a patrol officer, and after that be eligible for promotion under NYPD rules.