Some of the company's deferred products, including fixed-index annuity products, contain a first year interest bonus ranging from 1% to 7% depending upon the product, in addition to the base first year interest rate. Other products include a premium bonus ranging from 2% to 10% which is credited to the account balance when premiums are applied. These bonus rates are deferred in conjunction with other capitalized policy acquisition costs. The amount deferred to be amortized over future periods amounted to approximately $7.5 million, $17.9 million and $19.3 million for the years ended December 31, 2018, 2017 and 2016, respectively. Amortization of deferred sales inducements is included as a component of annuity contract interest as described later in this discussion of Annuity Operations.
The fluctuation in reported contract interest amounts for fixed-index annuities is driven by sales levels, the level of the business in force and the effect of positive or negative market returns of option values on projected interest credits. As noted above in the net investment income discussion, the amounts shown for contract interest for fixed-index annuities generally align with the derivative gains/(losses) included in net investment income as the impact of the market change of index options on asset values align closely with the movement of the embedded derivative liability held for these products. As noted in the discussion in the Consolidated Operations section, positive returns on expiring options during 2016 through 2018 led to collection of asset management fees by National Western which are subtracted from contract interest credited to policyholders. This serves to lessen the increase in contract interest expense relative to the option gains reported in the Company's net investment income. Asset management fees collected during 2018, 2017, and 2016 were $24.3 million, $31.8 million, and $6.4 million, respectively.
Since the Company's adoption of the GAAP guidance and accounting for other-than-temporary impairments due to credit loss versus non-credit loss, the Company has recognized on its securities held at December 31, 2018 a total of $0.6 million of other-than-temporary impairments of which $0.6 million were deemed credit related and recognized as realized investment losses in earnings, and $0.0 million that was deemed non-credit related impairments and recognized in other comprehensive income.
Market risk is the risk of change in market values of financial instruments due to changes in interest rates, currency exchange rates, commodity prices, or equity prices. The most significant market risk exposure for National Western is interest rate risk. Substantial and sustained increases and decreases in market interest rates can affect the profitability of insurance products and fair value of investments. The yield realized on new investments generally increases or decreases in direct relationship with interest rate changes. The fair values of fixed income debt securities correlate to external market interest rate conditions as market values typically increase when market interest rates decline and decrease when market interest rates rise. However, market values may fluctuate for other reasons, such as changing economic conditions, market dislocations or increasing event-risk concerns.
The Company relies on retained earnings for its capital resources as there is no long-term debt outstanding and the Company does not anticipate the need for any long-term debt in the near future. As of December 31, 2018, the Company maintained commitments for its normal operating and investment activities. In addition, the Company had pending an acquisition under a Stock Purchase Agreement executed October 3, 2018, which was subject to receiving the requisite regulatory approvals at December 31, 2018. As disclosed in Note (8) Subsequent Events in the Notes to Consolidated Financial Statements in Attachment A of Part IV Item 15. Exhibits and Financial Statement Schedules of this filing, the Company subsequently completed the announced acquisition on January 31, 2019 paying $205.5 million for the targeted companies.