Inclusion Criteria:

          -  Histologically confirmed, unresectable locally advanced or metastatic transitional
             cell carcinoma of the urothelium

          -  Stage IV disease at the start of first-line chemotherapy

          -  Measurable disease (per RECIST v1.1) prior to the start of first-line chemotherapy

          -  Prior first-line chemotherapy must have consisted of at least 4 cycles and no more
             than 6 cycles of gemcitabine + cisplatin and/or gemcitabine + carboplatin

          -  No evidence of progressive disease following completion of first-line chemotherapy
             (i.e., ongoing CR, PR, or SD per RECIST v1.1 guidelines )

        Exclusion Criteria:

          -  Prior adjuvant or neoadjuvant systemic therapy within 12 months of randomization

          -  Prior immunotherapy with IL-2, IFN-α, or an anti-PD-1, anti-PD-L1, anti-PD-L2,
             anti-CD137, or CTLA 4 antibody (including ipilimumab), or any other antibody or drug
             specifically targeting T-cell co-stimulation or immune checkpoint pathways

          -  Persisting toxicity related to prior therapy (Grade >1 NCI CTCAE v4.0); however,
             alopecia, sensory neuropathy (Grade 2 or less), or other (Grade 2 or less) adverse
             events not constituting a safety risk based on the investigator's judgement are
             acceptable.

          -  Patients with known symptomatic central nervous system (CNS) metastases requiring
             steroids

          -  Diagnosis of any other malignancy within 5 years prior to randomization, except for
             adequately treated basal cell or squamous cell skin cancer, carcinoma in situ of the
             breast or of the cervix, low grade prostate cancer on surveillance without any plans
             for treatment intervention, or prostate cancer that has been adequately treated with
             prostatectomy or radiotherapy and currently with no evidence of disease or symptoms.