Premiums. Gross written premiums increased by 13.6% to $6,573.7 million in 2018, compared to $5,788.5 million in 2017, reflecting a $637.0 million, or 16.2%, increase in our reinsurance business and a $148.2 million, or 8.0%, increase in our insurance business. The increase in reinsurance premiums was mainly due to the increases in treaty property and treaty casualty writings, rise in mortgage business and growth in Latin American business. The rise in insurance premiums was primarily due to increases in many lines of business, including casualty, energy and accident and health. Net written premiums increased by 84.7% to $5,031.9 million in 2018, compared to $2,723.8 million in 2017. The difference between the change in gross written premiums compared to the change in net written premiums is primarily due to the impact of changes in affiliated reinsurance contracts, particularly the non-renewal of the quota share agreement between Everest Re and Bermuda Re as of December 31, 2017. Effective January 1, 2018, Everest Re entered into an aggregate stop loss agreement with Bermuda Re. Premiums ceded to Bermuda Re in 2018 were $572.6 million compared with $2,219.4 million in 2017. Premiums earned increased by 148.2% to $4,839.1 million in 2018, compared to $1,949.6 million in 2017. The change in premiums earned relative to net written premiums is partially due to the impact of the non-renewal of the quota share contract between Everest Re and Bermuda Re and is also the result of timing; premiums are earned ratably over the coverage period whereas written premiums are recorded at the initiation of the coverage period.
2016. The decrease was primarily due to the impact of the decrease in premiums earned, the impact of higher reinstatement premiums in 2017 and the impact of changes in affiliated quota share contracts.
Corporate Expenses. Corporate expenses, which are general operating expenses that are not allocated to segments, have remained consistent at $11.0 million, $7.4 million and $8.3 million for the years ended December 31, 2018, 2017 and 2016, respectively.
Stockholder’s equity increased by $114.2 million to $5,412.7 million at December 31, 2017 from $5,298.6 million at December 31, 2016, principally as a result of $78.2 million of net income, $37.4 million of net foreign currency translation adjustments, $6.5 million of net benefit plan obligation adjustments and $0.3 million of share-based compensation transactions, partially offset by $8.2 million of net unrealized depreciation on investments, net of tax.
Incurred losses increased by 206.0% to $2,790.8 million in 2018 compared to $912.1 million in 2017. The increase was primarily due to a rise of $856.2 million in current year attritional losses mainly due to the impact of the increase in premiums earned resulting primarily from changes in affiliated reinsurance agreements, including the non-renewal of the quota share agreement with Bermuda Re as of December 31, 2017 and the implementation of an aggregate stop loss agreement with Bermuda Re as of January 1, 2018. The increase in incurred losses also resulted from an increase of $467.0 million in current year catastrophe losses and $454.3 million of unfavorable development on prior years catastrophe losses in 2018 primarily related to Hurricane Harvey, Irma and Maria and the 2017 California wildfires. The increases in loss estimates for Hurricane Harvey, Irma and Maria were mostly driven by re-opened claims reported in the second quarter of 2018 and loss inflation from higher than expected loss adjustment expenses and in particular, their impact on aggregate covers. The current year catastrophe losses of $952.2 million in 2018 primarily related to Hurricane Michael ($416.5 million), Camp wildfire ($297.0 million), Woolsey wildfire ($151.0 million),Hurricane Florence ($49.3 million), other 2018 California wildfires ($23.1 million), Typhoon Jebi ($6.5 million), Japan Floods ($5.5 million), Typhoon Trami ($2.0 million) and the U.S. winter storms ($1.3 million). The current year catastrophe losses of $485.2 million in 2017 related to Hurricane Irma
the South Africa Knysna fires ($9.9 million), Cyclone Debbie in Australia ($8.4 million), the Peru storms ($7.4 million) and Hurricane Harvey ($3.4 million).