Net investment income decreased $76 million or 13% in 2001, primarily attributable to the general decline in interest rates during 2001. Net investment income increased $93 million or 18% in 2000, primarily attributable to the combination of higher average investment yields (as a result of the increasing U.S. interest rate environment throughout most of 2000) and repositioning of the investment portfolio to include a higher proportion of fixed maturity securities.
Net premiums written decreased $890 million or 32% in 2001, primarily attributable to (1) higher levels of ceded losses under the aggregate excess retrocession program principally as a result of the events of September 11 and (2) the decision to exit certain lines of business and customer relationships as part of a reunderwriting initiative undertaken in 2000/2001, somewhat offset by general growth in premiums due to the combination of recent hardening of pricing within the overall property and casualty insurance/reinsurance industry. Net premiums written increased $241 million or 10% in 2000, primarily attributable to the combination of hardening pricing within the overall property and casualty insurance/reinsurance industry and the impact of a full year of operating activity for the March 1999 acquisition of Eagle Star Re. This increase was somewhat offset by the 2000 decision to exit certain lines of business and customer relationships as part of a reunderwriting initiative undertaken during the year.
The Company has a one-year $600 million revolving credit agreement with GE Capital Services which enables the Company to borrow from GE Capital Services at an interest rate per annum equal to GE Capital Services' cost of funds for a one year period. The agreement is automatically extended for successive terms of one year each unless terminated in accordance with terms of the agreement. The total amount outstanding on this credit facility, including accrued interest payable, was $108 million and $129 million as of December 31, 2001 and 2000, respectively.
The U.S. Securities and Exchange Commission requires that registrants provide information about potential effects of changes in interest rates and currency exchange. Although the rules offer alternatives for presenting this information, none of the alternatives is without limitations. The following discussion is based on so-called "shock tests," which model effects of interest rate and currency shifts on the reporting company. Shock tests, while probably the most meaningful analysis permitted, are constrained by several factors, including the necessity to conduct the analysis based on a single point in time and by their inability to include the complex market reactions that normally would arise from the market shifts modeled. While the following results of shock tests for interest rates and currencies may have some limited use as benchmarks, they should not be viewed as forecasts.
Impairment of investment securities results in a charge to operations when a market decline below cost is other than temporary. Management regularly reviews each investment security for impairment based on criteria that include the extent to which cost exceeds market value, the duration of that market decline and the financial health of and specific prospects for the issuer. The Company's investment securities amounted to approximately $22 billion at year-end 2001. Gross unrealized gains and losses included in that carrying amount related to debt securities were $317 million and $249 million, respectively. Gross unrealized gains and losses of equity securities were $19 million and $38 million, respectively. Of thoses securities whose carrying amount exceeds fair value at year-end 2001, and based on application of the Company's accounting policy for impairment, approximately $30 million of portfolio value is at risk of being charged to earnings in 2002. The Company actively performs comprehensive market research, monitors market conditions and segments its investments by credit risk in order to minimize impairment risks. Further information is provided in notes 2 and 4 to the consolidated financial statements and pages 19-21, which discusses the investment securities portfolio.