Wis. Stat. § 59.81: Cash flow, Milwaukee.

In counties having a population of 750,000 or more, the treasurer may be designated as the custodian for all cash received in an escrow, trust, bailment or safekeeping capacity by any other department of the county. This section is not applicable to the clerk of circuit court or any other depository specifically designated by a court of law or by a donor or other bailor even if the other depository retains control over such funds and the ultimate disposition. The treasurer may commingle this cash with general revenue cash and subject these funds to a common investment policy. Any interest earned on such investment reverts to the general fund of the county.