of our clients and our evaluation of risk/return characteristics, and trade credit and bond line, as we managed our European exposure base in light of existing economic conditions and changes in contract terms.
2013 versus 2012: Growth of 11% was primarily driven by our accident & health, liability, professional and property lines of business which were partially offset by decreases in marine and aviation. Our accident & health business has shown continued growth driven by new business opportunities. Liability growth was driven by select new business opportunities and, to a lesser extent, rate increases in the U.S. wholesale excess casualty market. Growth in the professional lines was largely due to continued expansion globally with new business and certain rate increases in the U.S. also contributing. Property premiums also contributed, as growth driven by new business and select rate increases more than offset reductions due to the non-renewal of certain catastrophe-exposed business written through MGAs. The segment's premium increases were partially offset by decreases in marine, which was impacted by non-renewal of certain business written through MGAs, and aviation which was primarily impacted by timing of renewals and certain rate decreases.
2012 versus 2011: Natural catastrophe and weather-related losses impacted the current accident year loss ratios for both periods. During 2012, we recognized estimated pre-tax net losses (inclusive of related premiums to reinstate our reinsurance protection) of $222 million for Storm Sandy and other U.S. natural catastrophe and weather-related events. Comparatively, during 2011, we recognized estimated pre-tax net losses (inclusive of related premiums to reinstate our reinsurance protection) of $156 million relating to a series of severe U.S. storms; floods in Thailand; the Japanese earthquake and tsunami; New Zealand earthquake; and Hurricane Irene and Tropical Storm Lee. Exclusive of these natural catastrophe and weather-related losses, our 2012 current accident year loss ratio improved relative to 2011 primarily due to:
General and Administrative Expense Ratio: General and administrative expenses increased in 2013, primarily driven by staffing increases, as we continue to build-out the segment's global platform. The impact of the increases on the general and administrative expense ratio was partially tempered by the growth in net premiums earned. The increase in the general and administrative expense ratio in 2012 compared to 2011, was driven by performance-based compensation expenses and staff increases.
At December 31, 2013, fixed maturities had a weighted average credit rating of AA- (2012: AA-), a book yield of 2.5% (2012: 2.6%) and an average duration of 3.3 years (2012: 3.0 years). The interest rate swap position introduced in 2013 reduced duration to 3.2 years. When incorporating short-term investments and cash and cash equivalents into this calculation (bringing the total to $13 billion), the weighted average credit rating would be AA- (2012: AA-) and duration (including interest rate swaps) would be 2.9 years (2012: 2.8 years).
The increase in the total fair value during 2013 was primarily due to incremental investments in CLO Debt securities. The CLO Debt is anticipated to reduce the impact of potential future increases in interest rates while not compromising the high credit quality of our fixed maturities portfolio. These securities were purchased primarily as new issues and all had ratings of AA- or better. The average duration and weighted average life of our ABS portfolio at December 31, 2013 was 0.6 years (2012: 0.9 years) and 4.0 years (2012: 3.6 years), respectively.
During 2013, we continued to add municipal debt securities, primarily within our taxable U.S. portfolios due to the attractive relative valuations available within this asset class.
The increase in gross unrealized losses on investment-grade securities is primarily due to the upward shift in sovereign yield curves. The severity range from 20-40% consists of emerging market debt securities which experienced a decline in pricing due to rising interest rates as well as adverse movements in foreign exchange rates.
At December 31, 2013, $33 million of the CLO fund commitment had been called. This is the primary reason for the increase in the fair value of our CLO - Equities during the year. In addition, our direct investments in CLO - Equities provided $27 million of net investment income during the year, offset by the receipt of $50 million of cash distributions. Included in these distributions were $24 million from two of our holdings which have entered the liquidation phase of their investment cycle and have begun returning capital to equity tranche holders.
the Lloyd's Facility Documents, the lender may exercise certain remedies, including the exercise of sole and exclusive control over pledged collateral.
Refer to the previous discussions of this business under ‘Reserving Process - Actuarial Analysis’ and ‘Reserving Process - Reserving for Credit and Political Risk Business’ above for a discussion of specific loss reserve issues related to this business. When considering prior accident year reserve development for this line of business, it is important to note that the multi-year nature of the credit business distorts loss ratios when a single accident year is considered in isolation. In recent years, the average term of these contracts has been four to five years. The premiums we receive are generally earned evenly over the contract term, thus spanning multiple accident years. In contrast, losses incurred on these contracts, which can be characterized as low in frequency and high in severity, are reflected in a single accident year.
At December 31, 2013, the Company is not party to any off-balance sheet arrangements, as defined by Item 303(a)(4) of Regulation S-K to which an entity unconsolidated with the Company is a party that management believes is reasonably likely to have a current or future effect on its financial condition, revenues or expenses, results of operations, liquidity, capital expenditures or capital resources that the Company believes is material to investors.
We believe that showing net income available to common shareholders exclusive of net realized gains (losses), foreign exchange losses (gains) and losses on repurchase of preferred shares reflects the underlying fundamentals of our business. In addition, we believe that this presentation enables investors and other users of our financial information to analyze performance in a manner similar to how our management analyzes the underlying business performance. We also believe this measure follows industry practice and, therefore, facilitates comparison of our performance with our peer group. We believe that equity analysts and certain rating agencies that follow us, and the insurance industry as a whole, generally exclude these items from their analyses for the same reasons.
Any subsequent differences arising on insurance and reinsurance premium estimates are recorded in the period they are determined.
Direct lending funds: Seek to achieve attractive risk-adjusted returns, including current income generation, by investing in funds which provide financing directly to borrowers.
Observable inputs: market prices for similar instruments, notional, option strike, term to expiry, contractual limits;
On November 15, 2004, AXIS Capital issued $500 million aggregate principal amount of 5.75% senior unsecured debt (the “5.75% Senior Notes”) at an issue price of 99.785%, generating net proceeds of $496 million. Interest on the 5.75% Senior Notes is payable semi-annually in arrears on June 1 and December 1 of each year, beginning on June 1, 2005. Unless previously redeemed, the 5.75% Senior Notes will mature on December 1, 2014.
During 2013, we incurred a compensation cost of $57 million (2012: $68 million; 2011: $39 million) in respect of all restricted stock and restricted stock units, and recorded tax benefits thereon of $10 million (2012: $8 million; 2011: $7 million). Compensation costs incurred in 2012 includes $20 million of accelerated and incremental expenses associated with the transition in our senior leadership. The retirement of one senior leader resulted in the modification of that leader's outstanding restricted stock awards to eliminate the previously existing service condition. The unanticipated termination without cause of another senior officer lead to the early satisfaction of employment obligations and resulted in the service condition associated with that officer's outstanding restricted stock awards being fully satisfied.
During 2013, we also granted 847,425 restricted stock units that will settle in cash rather than shares when the awards ultimately vest. At December 31, 2013, the corresponding liability for cash-settled awards, included in other liabilities on the Consolidated Balance Sheets, was $9 million (2012: $nil).
The following is a summary of stock options outstanding and exercisable at December 31, 2013 and related activity for the year ended:
Deferred income taxes reflect the tax impact of temporary differences between the carrying amounts of assets and liabilities for financial reporting and income tax purposes. The significant components of our deferred tax assets and liabilities were as follows: