Net investment income. In 2015, we generated net investment income of $185.5 million, a decrease of 2.5% on the prior year (2014 - $190.3 million; 2013 - $186.4 million). The decrease was primarily due to lower reinvestment rates and declining book yields from fixed income securities partially offset by $20.1 million of dividend income from our global equity securities portfolio in 2015 compared with $17.1 million in 2014 and $12.6 million in 2013. The increase in investment income in 2014 compared to 2013 was due primarily to a higher investment balance compared to 2013.
These factors are incorporated in the recommended reserve range from which management selects its best point estimate. We take all reasonable steps to ensure that we utilize all appropriate information and actuarial techniques in establishing our IBNR reserves. However, given the uncertainty in establishing claims liabilities, it is likely that the final outcome will prove to be different from the original provision established at the balance sheet date. Changes to our previous estimates of prior period loss reserves impact the reported calendar year underwriting results by worsening our reported results if the prior year reserves prove to be deficient or improving our reported results if the prior year reserves prove to be redundant. As at December 31, 2015, a 5% change in the gross reserve for IBNR losses would have equated to a change of approximately $142.1 million in loss reserves which would represent 42.1% of income before income tax for the twelve months ended December 31, 2015. As at December 31, 2014, a 5% change in the gross reserve for IBNR losses would have equated to a change of approximately $135.7 million in loss reserves which would represent 36.9% of income before income tax for the twelve months ended December 31, 2014. A 5% change in our net loss reserves equates to $229.2 million and represents 6.4% of shareholders’ equity at December 31, 2015.
Insurance. Net reserve releases of $5.1 million in 2014 were mainly attributable to our property and casualty lines but were partially offset by a $38.0 million net strengthening in the marine, aviation and energy lines, in particular in our construction
The fees and interest rates on the loans and the fees on the letters of credit payable by the Borrowers under the Credit Agreement are based upon the credit ratings for Aspen Holdings’ long-term unsecured senior debt by S&P and Moody’s. In addition, the fees for a letter of credit vary based upon whether the applicable Borrower has provided collateral (in the form of cash or qualifying debt securities) to secure its reimbursement obligations with respect to such letter of credit.
Average equity, a non-U.S. GAAP financial measure, is calculated by the arithmetic average on a monthly basis for the stated periods excluding (i) preference shares, (ii) after-tax unrealized appreciation or depreciation on investments and (iii) the average after-tax unrealized foreign exchange gains and losses. Unrealized appreciation (depreciation) on investments is primarily the result of interest rate movements and the resultant impact on fixed income securities, and unrealized appreciation (depreciation) on foreign exchange is the result of exchange rate movements between the U.S. Dollar and the functional