Although our profit margins continue to be pressured by the impact of the low interest rate environment on our net investment income yields, our invested asset quality remains strong. The net unrealized gain on our fixed maturity securities was $4.7 billion at December 31, 2016, compared to $3.7 billion at December 31, 2015, with the increase due to a decline in credit spreads partially offset by an increase in U.S. Treasury rates during 2016. The earned book yield on our investment portfolio was 5.30 percent for 2016 compared to a yield of 5.40 percent for 2015.
The following are our current assumptions regarding the length of our amortization periods, the approximate DAC balance that remains at the end of years 3, 10, and 15 as a percentage of the cost initially deferred, and our DAC balances as of December 31, 2016 and 2015.
Overall benefits experience was favorable in 2016 relative to the prior periods, with a consolidated benefit ratio of 83.1 percent in 2016 compared to 83.9 percent in 2015 and 93.8 percent in 2014. Excluding the 2014 reserve adjustment, the benefit ratio for 2014 was 84.8 percent. The underlying benefits experience for each of our operating business segments is discussed more fully in "Segment Results" contained herein in this Item 7.
The low interest rate environment continues to place pressure on our profit margins by impacting net investment income yields as well discount rates on our insurance liabilities. Our net investment income may be impacted, either favorably or unfavorably, by fluctuations in miscellaneous investment income. As part of our continued pricing discipline and our reserving strategy, we continuously monitor emerging interest rate experience and adjust our pricing and reserve discount rates, as appropriate. We believe the group long-term disability reserve discount rate reduction during 2016 provides a strong balance sheet position for 2017. We currently expect our group disability benefit ratio for full year 2017 to be generally consistent with the level of 2016, although unfavorable economic conditions could lead to a higher rate of claim incidence or lower levels of claim recoveries. We continuously monitor key indicators to assess our risks and attempt to adjust our business plans accordingly.
Sales were higher in 2015 compared to 2014 due to growth in new and existing customer account sales. Commercial market sales increased compared to 2014 due to higher new and existing customer account sales in the core market and higher existing customer account sales in the large case market. The growth in our public sector market for 2015 was attributable to new and existing customer account sales. The number of new accounts increased 10.0 percent in 2015 compared to 2014, and the average new case size decreased 7.5 percent.
The following two tables show the length of time our investment-grade and below-investment-grade fixed maturity securities had been in a gross unrealized loss position as of December 31, 2016 and at the end of the prior four quarters. The relationships of the current fair value to amortized cost are not necessarily indicative of the fair value to amortized cost relationships for the securities throughout the entire time that the securities have been in an unrealized loss position nor are they necessarily indicative of the relationships after December 31, 2016. The decrease in the unrealized loss on fixed maturity securities during 2016 was due primarily to a decrease in credit spreads, partially offset by an increase in U.S. Treasury rates which occurred during the period.
Approximately 76.3 percent of our fixed maturity securities were valued based on prices from pricing services that generally use observable inputs such as prices for securities or comparable securities in active markets in their valuation techniques. These assets were classified as Level 2. Level 2 assets or liabilities are those valued using inputs (other than prices included in Level 1) that are either directly or indirectly observable for the asset or liability through correlation with market data at the measurement date and for the duration of the instrument's anticipated life.
At December 31, 2016 and 2015, we held no mortgage loans that were greater than 90 days past due regarding principal and/or interest payments.
In 2014, we amended our U.S. qualified defined benefit pension plan to allow a limited-time offer of benefit payouts to eligible former employees with a vested right to a pension benefit. The offer provided eligible former employees, regardless of age, with an option to elect to receive a lump-sum settlement of his or her entire accrued pension benefit in December 2014 or to elect receipt of monthly pension benefits commencing in January 2015. For those who elected to receive lump-sum settlements, we made payments totaling $214.5 million from plan assets in December 2014. We recognized a before-tax settlement loss of $64.4 million in earnings during 2014, with a corresponding reduction in the unrecognized actuarial loss included in accumulated other comprehensive income that pertained to the settled benefit obligation.