Incurred losses and LAE increased by 44.0% to $4,522.6 million in 2017, compared to $3,139.6 million in 2016, primarily due to an increase of $1,114.6 million in current year catastrophe losses, an increase in current year attritional losses of $266.4 million, mainly due to the impact of the increase in premiums earned and $56.6 million of less favorable development on prior years catastrophe losses in 2017 compared to 2016. These increases were partially offset by an additional $54.7 million of favorable development on prior years attritional losses in 2017 compared to 2016. The $263.4 million of favorable development on prior years attritional losses in 2017 was mainly comprised of $207.1 million of favorable development on reinsurance business, primarily related to property and short tail business in the United States and Bermuda, and $56.4 million of favorable development on insurance business, mainly related to workers compensation business. The $1,502.5 million of current year catastrophe losses in 2017 related to Hurricane Irma ($558.1 million), Hurricane Maria ($361.7 million), Hurricane Harvey ($316.7 million), the Northern California wildfires ($149.9 million), the Mexico City earthquake ($31.0 million), the South Africa Knysna fires ($23.7 million), Cyclone Debbie in Australia ($22.1 million), the Peru storms ($14.9 million), the 2017 US Midwest storms ($12.9 million) and the Southern California wildfires ($11.6 million). The $387.9 million of current year catastrophe losses in 2016 related to Hurricane Matthew ($135.0 million), the Fort McMurray Canada wildfire ($115.8 million), 2016 U.S. storms ($51.6 million), the Ecuador earthquake
Gross written premiums decreased by 7.8% to $1,230.7 million in 2016 compared to $1,334.2 million in 2015, primarily due to declines in Latin American, Middle East and Asian business and the negative impact of $40.7 million from the movement of foreign exchange rates. Net written premiums decreased by 10.4% to $1,082.7 million in 2016 compared to $1,209.0 million in 2015. The difference between the change in gross written premiums compared to the change in net written premiums is primarily due to varying utilization of reinsurance related to the quota share contracts. Premiums earned decreased 10.6% to $1,119.1 million in 2016 compared to $1,251.1 million in 2015. The change in premiums earned relative to net written premiums is primarily the result of timing; premiums are earned ratably over the coverage period whereas written premiums are recorded at the initiation of the coverage period.
For the portion needed to satisfy global outstanding liabilities, we generally invest in taxable and tax-preferenced fixed income securities with an average credit quality of Aa3. For the U.S. portion of this portfolio, our mix of taxable and tax-preferenced investments is adjusted periodically, consistent with our current and projected U.S. operating results, market conditions and our tax position. This global fixed maturity securities portfolio is externally managed by an independent, professional investment manager using portfolio guidelines approved by internal management.
The Everest International Credit Facility requires Group to maintain a debt to capital ratio of not greater than 0.35 to 1 and to maintain a minimum net worth. Minimum net worth is an amount equal to the sum of $5,326.0 million (70% of consolidated net worth as of December 31, 2015), plus 25% of consolidated net income for each of Group's fiscal quarters, for which statements are available ending on or after January 1, 2015 and for which net income is positive, plus 25% of any increase in consolidated net worth of Group during such period attributable to the issuance of ordinary and preferred shares, which at December 31, 2017, was $5,867.7 million. As of December 31, 2017, the Company was in compliance with all Everest International Credit Facility requirements.