Workers' Compensation Segment. Many factors affect the ultimate losses incurred for our workers' compensation coverages (9% of our consolidated gross reserve for losses and loss adjustment expenses for the year ended December 31, 2015), including, but not limited to, the type and severity of the injury, the age and occupation of the injured worker, the estimated length of disability, medical treatment and related costs, and the jurisdiction and workers' compensation laws of the injury occurrence. We use various actuarial methodologies, described below, in developing our workers’ compensation reserve, combined with a review of the exposure base generally based upon payroll. For the current accident year, given the lack of seasoned information, the different actuarial methodologies produce results with significant variability; therefore, more emphasis is placed on supplementing results from the actuarial methodologies with trends in exposure base, medical expense inflation, general inflation, severity, and claim counts, among other things, to select an expected loss ratio.
Because of the number of securities we own and the complexity of developing accurate fair values, we utilize multiple independent pricing services to assist us in establishing the fair value of individual securities. The pricing services provide fair
Certain of our investments, in accordance with GAAP for the type of investment, are measured using methodologies other than fair value. At December 31, 2015 these investments represented approximately 7% of total investments, and are detailed in the following table. Additional information about these investments is provided in Notes 3 and 4 of the Notes to Consolidated Financial Statements.
Policy acquisition costs (primarily commissions, premium taxes and underwriting salaries) which are directly related to the successful acquisition of new and renewal premiums are capitalized as DPAC and charged to expense, net of ceding commissions earned, as the related premium revenue is recognized. We evaluate the recoverability of our DPAC at the segment level each reporting period, and any amounts estimated to be unrecoverable are charged to expense in the current period. As of December 31, 2015 we have not determined that any amounts are unrecoverable.
We generally reinsure risks under treaties (our excess of loss reinsurance arrangements) pursuant to which the reinsurers agree to assume all or a portion of all risks that we insure above our individual risk retention levels, up to the maximum individual limits offered. These arrangements are negotiated and renewed annually. Renewal dates for our professional liability, medical technology liability and workers' compensation treaties are October 1, January 1 and May 1, respectively. As of October 1, 2015 our professional liability treaty renewed with more favorable terms and pricing than the previous agreement. Our workers' compensation and medical technology liability treaties also renewed at a more favorable rate than the previous agreements upon the latest renewal of each. The significant terms of our excess of loss reinsurance arrangements are detailed in the following table.
We have available a Revolving Credit Agreement which may be used for general corporate purposes, including, but not limited to, short-term working capital, share repurchases as authorized by the Board, and support for other activities we enter into in the normal course of business. On June 19, 2015 we executed an amendment to our Revolving Credit Agreement which extended the expiration to June 2020 and expanded the permitted borrowings from $200 million to $250 million by adding an accordion feature, which, if subscribed to successfully, would make another $50 million available for use. In addition, the amendment included other revisions related to pricing and financial covenants, which can be found in more detail in Note 10 of the Notes to Consolidated Financial Statements. At December 31, 2015 we had a fully secured outstanding borrowing of $100 million under the Revolving Credit Agreement, which can be repaid as early as April 2016, but can also be deferred until expiration of the Revolving Credit Agreement. We are in compliance with the financial covenants of the Revolving Credit Agreement.
Ceded premiums written for the years ended December 31, 2015 and 2014 were comprised as follows:
New business, audit premium, retention and renewal price changes for both the traditional business and the alternative market business for 2015 and 2014 are shown in the table below:
During 2015, we began investing in historic tax credit partnerships. Tax credits provided by the underlying projects of these partnerships are typically available in the tax year in which the project is put into active service, whereas the tax credits provided by qualified affordable housing tax credit partnerships, which make up the majority of our tax credit investments, are provided over approximately a ten year period. The increase in the amount of tax credits recognized in 2015 is primarily due to our investment in historic tax credit partnerships.
Tax-exempt income decreased in 2015 but had an increased effect on our effective tax rate because of an even greater decline in pre-tax income during 2015. The increased impact of the tax credits on the tax rate was primarily due to lower pre-tax income in 2015, but also reflected an increased amount of credits. Tax credits recognized for 2015 were $22.4 million as compared to $17.9 million for 2014.
The remaining increase in the current accident year ceded premiums ratio for the year ended December 31, 2014 was primarily attributable to the increase in ceded premiums written under the quota share arrangement with Syndicate 1729 and our shared risk arrangements, as previously discussed. Additionally, premium volume from retained coverages was lower in 2014 than in 2013, which reduced gross premiums written but had no effect on ceded premiums written, and thus increased the ratio.
Interest expense increased during 2014 as compared to 2013 primarily due to the issuance of unsecured senior notes in the fourth quarter of 2013 which carry a higher interest rate and are greater in amount than our average borrowing outstanding in 2013. Our weighted average outstanding debt approximated $250 million for the year ended December 31, 2014 as compared to $119 million for the year ended December 31, 2013.