Under the Tax Act, the federal statutory rate has been changed from 35% to 21%, effective for 2018. Although this change has impacted net income at December 31, 2017, as discussed earlier, we exclude the initial impact to deferred income taxes from a change in tax laws from non-GAAP operating income. See Note 5 to our consolidated financial statements included in item 8 for additional information regarding the Tax Act.
The guidance was adopted prospectively. Adoption resulted in a federal income tax benefit of $0.6 million ($0.02 per basic and diluted common share) for the year ended December 31, 2017. Prior periods were not restated.
Amount represents credit-related losses for other investments, real estate and fixed maturities written down to fair value through income. Also included are impairment losses related to investments accounted for under the equity method of accounting, which are included in securities and indebtedness of related parties within our consolidated balance sheets.
At December 31, 2017, affidavits of deposits covering investments with a carrying value totaling $8,066.4 million were on deposit with state agencies to meet regulatory requirements. Fixed maturities with a carrying value of $443.9 million were on deposit with the Federal Home Loan Bank of Des Moines (FHLB) as collateral for funding agreements.
Level 1 equity securities consist of mutual funds that are actively traded, allowing us to use current market prices as an estimate of their fair value.
Beginning in 2017, accounting guidance requires tax benefits of equity-based compensation to be recorded through net income rather than directly to stockholders' equity. Accordingly, we do not expect to have a provision for taxes on Class A and B common stock for years after 2016. See Note 1 for further discussion of this accounting change.
Periodically, we declare a special cash dividend payable to our Class A and Class B common shareholders. We have paid the following special dividends during the last three years: $1.50 per share in 2017 totaling $37.4 million, $2.00 per share in 2016 totaling $49.7 million and $2.00 per share in 2015 totaling $49.5 million.
For descriptions of the underfunded portion of our postretirement benefit plans, see Note 8 - Other Retirement Plans, and for certain other defined benefit plans, see Note 8 - Defined Benefit Pension Plans.
Prior to 2012, we granted stock options for Class A common stock to officers and employees, which have a contractual term of 10 years and vest over a period up to five years, contingent upon continued employment with us. Prior to 2009, we also granted stock options for Class A common stock to directors, which were fully vested upon grant and had a contractual term that varied with the length of time the director remained on the Board, up to 10 years. The exercise price for all options is equal to the fair value of the common stock on the grant date.
We lease our home office properties under a 10-year operating lease, which expires in 2021, from a wholly-owned subsidiary of the IFBF. Future remaining minimum lease payments under this lease, as of December 31, 2017, are as follows: 2018 - $2.2 million, 2019 - $2.2 million, 2020 - $2.2 million and 2021 - $2.2 million. Rent expense for the lease totaled $4.1 million in 2017 and 2016 and $4.0 million in 2015. These amounts are net of $0.2 million in 2017, 2016 and 2015 in amortization of a deferred gain on the exchange of our home office properties for common stock in 1998. The remaining unamortized deferred gain totaled $0.7 million at December 31, 2017 and $0.9 million at December 31, 2016.