Premiums. Gross written premiums decreased by 1.0% to $2,125.8 million in 2016 from $2,147.9 million in 2015, primarily due to a decrease in treaty property business, partially offset by an increase in treaty casualty business. Net written premiums increased by 6.2% to $1,970.6 million in 2016 compared to $1,855.9 million in 2015. The difference between the change in gross written premiums compared to the change in net written premiums is primarily due to the assumption of the crop business due to the sale of Heartland and a concurrent new crop reinsurance contract. Premiums earned increased 6.1% to $2,072.2 million in 2016, compared to $1,952.7 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.
With respect to asbestos only, at December 31, 2016, we had net asbestos loss reserves of $303.6 million, or 95.2%, of total net A&E reserves, all of which was for assumed business.
Our shareholders' equity increased to $7,608.6 million as of December 31, 2015 from $7,451.1 million as of December 31, 2014. This increase was the result of $977.9 million of net income, share-based compensation transactions of $34.8 million and $11.9 million of net benefit plan obligation adjustments, partially offset by repurchases of 2.3 million common shares for $400.1 million, $180.4 million of unrealized depreciation on investments, net of tax, $175.1 million of shareholder dividends and $111.5 million of net foreign currency translation adjustments.
Our financial strength ratings as determined by A.M. Best, Standard & Poor's and Moody's are important as they provide our customers and investors with an independent assessment of our financial strength using a rating scale that provides for relative comparisons. We continue to possess significant financial flexibility and access to debt and equity markets as a result of our financial strength, as evidenced by the financial strength ratings as assigned by independent rating agencies. See also ITEM 1, Business - "Financial Strength Ratings".
Management estimates that the projected net economic loss from its largest 100-year event in a given zone represents approximately 11% of its December 31, 2016 shareholders' equity. Economic loss is the PML exposure, net of third party reinsurance, reduced by estimated reinstatement premiums to renew coverage and estimated income taxes. The impact of income taxes on the PML depends on the distribution of the losses by corporate entity, which is also affected by inter-affiliate reinsurance. Management also monitors and controls its largest PMLs at multiple points along the loss distribution curve, such as loss amounts at the 20, 50, 100, 250, 500 and 1,000 year return periods. This process enables management to identify and control exposure accumulations and to integrate such exposures into enterprise risk, underwriting and capital management decisions.
The following tables present certain financial statement line items as previously reported in 2015 and 2014, the effect on those line items due to not consolidating the segregated accounts of Mt. Logan Re, in accordance with the newly adopted accounting policy and the line items as currently reported within the financial statements.
Interest Rate Risk. Our $17.5 billion investment portfolio, at December 31, 2016, is principally comprised of fixed maturity securities, which are generally subject to interest rate risk and some foreign currency exchange rate risk, and some equity securities, which are subject to price fluctuations and some foreign exchange rate risk. The overall economic impact of the foreign exchange risks on the investment portfolio is partially mitigated by changes in the dollar value of foreign currency denominated liabilities and their associated income statement impact.
The tables below display the potential impact of market value fluctuations and after-tax unrealized appreciation on our fixed maturity portfolio (including $431.5 million of short-term investments) for the period indicated based on upward and downward parallel and immediate 100 and 200 basis point shifts in interest rates. For legal entities with a U.S. dollar functional currency, this modeling was performed on each security individually. To generate appropriate price estimates on mortgage-backed securities, changes in prepayment expectations under different interest rate environments were taken into account. For legal entities with a non-U.S. dollar functional currency, the effective duration of the involved portfolio of securities was used as a proxy for the market value change under the various interest rate change scenarios.