Any city or town now or hereafter operating an employees' pension system with the approval of the board otherwise responsible for management of its respective funds may invest, reinvest, manage, contract, sell, or exchange investments acquired. Investments shall be made in accordance with investment policy duly established and published by the board. In discharging its duties under this section, the board shall act with the care, skill, prudence, and diligence under the circumstances then prevailing that a prudent person acting in a like capacity and familiar with such matters would use in the conduct of an enterprise of a like character and with like aims; shall diversify the investments of the employees' pension system so as to minimize the risk of large losses; and shall act in accordance with the documents and instruments governing the employees' pension system, insofar as such documents and instruments are consistent with the provisions of this title.