Net income increased 162% to $458.0 million in 2018 and 110% to $174.6 million in 2017 from $83.2 million in 2016. Net income, in general, has been positively impacted by the growth in the volume of business in force and the investment spread earned on this business. The average amount of annuity account balances outstanding (net of annuity liabilities ceded under coinsurance agreements) increased 6% to $49.9 billion for the year ended December 31, 2018 compared to $46.8 billion in 2017 and 8% for the year ended December 31, 2017 compared to $43.5 billion in 2016. Our investment spread measured in dollars was $1.2 billion, $1.2 billion, and $1.0 billion for the years ended December 31, 2018, 2017 and 2016, respectively. As previously mentioned, our investment spread has been negatively impacted by the extended low interest rate environment (see Net investment income) and the increase in our aggregate cost of money due to an increase in option costs for certain index strategies we have been experiencing for the last several quarters.
Amortization of deferred policy acquisition costs, in general, has been increasing each year due to the growth in our annuity business and the deferral of policy acquisition costs incurred with respect to sales of annuity products. The increases in amortization from these factors have been affected by amortization associated with (1) fair value accounting for derivatives and embedded derivatives utilized in our fixed index annuity business, (2) net realized gains (losses) on investments and net OTTI losses recognized in operations, and (3) changes in litigation reserves. As discussed above, fair value accounting for derivatives and embedded derivatives utilized in our fixed index annuity business creates differences in the recognition of revenues and expenses from derivative instruments including the embedded derivative liabilities in our fixed index annuity contracts. Amortization of deferred policy acquisition costs is summarized as follows:
Salary and benefits expense increased in 2018 as compared to 2017 as a result of an increase in salary and benefits of $5.4 million due to an increased number of employees related to our growth and an increase of $6.8 million related to expense recognized under our equity and cash incentive compensation programs ("incentive compensation programs"). The increases in expense for our incentive compensation programs were primarily due to increases in the actual and expected payouts due to a larger number of employees participating in the programs, higher potential payouts for certain employees participating in the programs and an increase in the percentage of restricted stock units that were earned or expected to be earned. In addition, salary and benefits for 2017 reflected a benefit of $1.3 million related to a retirement agreement with our former executive chairman.
At each balance sheet date, we identify invested assets which have characteristics (i.e. significant unrealized losses compared to amortized cost and industry trends) creating uncertainty as to our future assessment of an other than temporary impairment. As part of this assessment, we review not only a change in current price relative to its amortized cost but the issuer's current credit rating and the probability of full recovery of principal based upon the issuer's financial strength. Specifically for corporate issues we evaluate the financial stability and quality of asset coverage for the securities relative to the term to maturity for the issues we own. A security which has a 20% or greater change in market price relative to its amortized cost and/or a possibility of a loss of principal will be included on a list which is referred to as our watch list. We exclude from this list securities with unrealized losses which are related to market movements in interest rates and which have no factors indicating that such unrealized losses may be other than temporary as we do not intend to sell these securities and it is more likely than not we will not have to sell these securities before a recovery is realized. In addition, we exclude our residential and commercial mortgage backed securities as we monitor all of our residential and commercial mortgage backed securities on a quarterly basis for changes in default rates, loss severities and expected cash flows for the purpose of assessing potential other than temporary impairments and related credit losses to be recognized in operations. At December 31, 2018, the amortized cost and fair value of securities on the watch list (all fixed maturity securities) are as follows:
Energy, Industrials and Materials: The decline in the value of these securities relates to continued operational pressure due to past declines in certain commodity prices specific to their businesses. The decline in these commodity prices creates financial challenges as the companies had to realign operations to accommodate the new environment. These issuers will be stressed greater than the average company due to their price sensitivity and the specific position they hold in the supply chain. While values have declined, improving commodity prices should continue to provide better financial performance for these companies. We recognized an other than temporary impairment on two of these issuers during 2018 due to our evaluation of the operating performance and the credit worthiness of each issuer. While the remaining issuers have seen their financial and profitability profile weakened, we have determined that the remaining securities were not other than temporarily impaired due to our evaluation of the operating performance and the credit worthiness of the issuer.
We evaluate our mortgage loan portfolio for the establishment of a loan loss allowance by specific identification of impaired loans and the measurement of an estimated loss for each individual loan identified. A mortgage loan is impaired when it is probable that we will be unable to collect all amounts due according to the contractual terms of the loan agreement. If we determine that the value of any specific mortgage loan is impaired, the carrying amount of the mortgage loan will be reduced to its fair value, based upon the present value of expected future cash flows from the loan discounted at the loan's effective interest rate, or the fair value of the underlying collateral less estimated costs to sell.