(5) We adjusted certain prior period amounts to reflect our adoption of a new accounting standard in 2012. See Note 1, “General Information and Significant Accounting and Reporting Policies - Accounting Guidance Adopted in 2012” to the Consolidated Financial Statements. Adoption of this standard impacted our consolidated financial statements, as follows:
Net investment income, which is included in our Investing segment, increased 5% in 2012 and 4% in 2011 due to growth in our investment portfolio, partially offset by the effect of reduced yields. Our fixed maturity securities portfolio increased 7% in 2012 and 13% in 2011, from $5.2 billion at December 31, 2010 to $5.9 billion at December 31, 2011 and $6.3 billion at December 31, 2012. In addition, we added publicly traded equity securities to our portfolio in 2012 and held $284.6 million at December 31, 2012. The growth in investments resulted primarily from cash flow from operations and an increase of $105.6 million in the net unrealized gain on our available for sale securities during 2012. Our investment expense increased in 2012 due to growth in the portfolio and management expenses for the equity securities.
U.S. D&O’s 2012 net loss ratio includes the impact of using DFP’s higher ultimate loss ratio in 2012 for DFP’s underwriting year 2011 premium that earned in 2012. The 2011 net loss ratio for U.S. D&O included the impact of DFP’s adverse development, partially offset by the favorable development for the U.S. D&O line of business. International D&O’s lower loss ratios in 2012 and 2011, compared to 2010, directly related to favorable development in those years.
Our International segment pretax earnings were impacted in all years by net catastrophe losses. The 2012 losses primarily related to Superstorm Sandy and impacted our property treaty, property (direct and facultative) and energy lines of business. In 2011, we experienced losses from catastrophes in Japan, New Zealand, Australia, the United States and Denmark. The 2011 catastrophic events impacted our energy and property treaty lines of business, as well as our property and accident and health lines (both included in Other). The 2010 catastrophe losses occurred in our property treaty and property lines, primarily related to the Chile earthquake. We reinsured a portion of our exposure to these catastrophic events and incurred net reinstatement premium for continued reinsurance coverage, which reduced 2011 net written and net earned premium. The following table summarizes the segment’s accident year catastrophe losses, as well as the impact on key metrics:
The segment’s increase in gross written, net written and net earned premium in 2012 was driven by higher writings in the energy, property treaty and property lines of business, related to the favorable pricing environment for these products. Net written and net earned premium also increased in 2012 due to lower reinstatement premium related to catastrophic events in 2012, compared to 2011. The increase in premium in 2011 principally related to our property treaty business, which we began to write in late 2009. In addition, in 2011, we wrote more energy business due to industry rate increases and expansion of our wind storm aggregates and retained a higher percentage of this business. Premium reported in Other decreased in 2011, primarily because we incurred $8.0 million of reinstatement premium related to the 2011 catastrophe losses and wrote less property business compared to 2010, which included short-tail business that was substantially reinsured.
In 2012, 2011 and 2010, our domestic and foreign insurance companies paid HCC dividends of $262.4 million, $248.2 million and $285.7 million, respectively. The payment of dividends by our insurance companies is subject to regulatory