The board of trustees shall invest the funds of the retirement system in investments of the nature which individuals of prudence, discretion, and intelligence acquire or retain in dealing with the property of another. Such investments shall not be made for speculation but for investment, considering the probable safety of their capital as well as the probable income to be derived. The board of trustees shall not purchase investments on margin or enter into any futures contract or other contract obligation which requires the payment of margin or enter into any similar contractual arrangement which may result in losses in excess of the amount paid or deposited with respect to such investment or contract, unless such transaction constitutes a hedging transaction or is incurred for the purpose of portfolio or risk management for the funds and investments of the system. Prior to January 1, 2017, the board of trustees may write covered call options or put options. Prior to January 1, 2017, the board of trustees shall establish written guidelines for any such option, purchase, or contract obligation. Any such option, purchase, or contract obligation shall be governed by the prudent investment rule stated in this section for investment of the funds of the system. The board of trustees may lend any security if cash, United States Government obligations, or United States Government agency obligations with a market value equal to or exceeding the market value of the security lent are received as collateral. Prior to January 1, 2017, if shares of stock are purchased under this section, all proxies may be voted by the board of trustees prior to January 1, 2017. As of January 1, 2017, the funds of the retirement system shall be invested solely by the council and the state investment officer in accordance with the Nebraska State Funds Investment Act. The state investment officer may lend securities and vote proxies in accordance with the standard set forth in section 72-1246.