Interest margins consist of net investment income earned on universal life, fixed indexed annuities and other policyholder funds, gains on options purchased to fund index credits less the interest or index credits applied to policyholders on those funds. Interest margins also include investment income on assets supporting the Company’s surplus.
Income tax benefit increased $44.8 million for the year ended December 31, 2015 compared with the year ended December 31, 2014 as a result of taxable losses in the insurance subsidiaries that was generated in 2015. These losses generate a refund of historically paid income tax expense.
Overall, mortality was unfavorable compared with expectations in 2015 in both the open and the closed block, while overall mortality was favorable compared with expectations in 2014. The impact of mortality on earnings is driven by mortality outside of the closed block, as any earnings in the closed block inure to the policyholders through changes in policyholder dividends. Net death claims were $783.0 million and $550.6 million for the years ended December 31, 2015 and 2014, respectively, with net claims outside of the closed block of $366.9 million and $266.4 million for the same periods. The worse mortality outside of the closed block was primarily in the universal life business, where our long-term assumptions are supported by mortality experience observed over longer periods.
See Note 11 to our consolidated financial statements under “Item 8: Financial Statements and Supplementary Data” in this Form 10-K for additional information on derivative instruments.
We sponsor defined benefit pension plans for our employees. For U.S. GAAP accounting purposes, we assumed a 7.5% long-term rate of return on plan assets in the most recent valuations, performed as of December 31, 2015. To the extent there are deviations in actual returns, there will be changes in our projected expense and funding requirements. As of December 31, 2015, the projected benefit obligation for our funded and unfunded defined benefit plans was in excess of plan assets by $179.1 million and $142.7 million, respectively. We made contributions totaling $0.0 million and $11.8 million to the pension plans during 2015 and 2014, respectively. We expect to make no contributions over the next 12 months.
Cash used for investing activities decreased $396.1 million during the year ended December 31, 2015 compared to the year ended December 31, 2014. This decrease was primarily a result of increased sales of available-for-sale debt securities, driven by an increase in death benefits on universal life and traditional life products. Partially offsetting this decrease was an increase in purchases of short-term investments.
As a result of discussions with its regulators related to the intercompany reinsurance treaty between Phoenix Life and PHL Variable executed in the second quarter of 2015, the Company’s operating subsidiaries were de-stacked, effective July 1, 2015. The impact of the de-stack on Phoenix Life’s capital and surplus was $262.2 million, which included $228.2 million for the reduction in subsidiary investment and $34.0 million for the related decrease in admitted deferred tax assets.