A key factor in achieving our targeted combined ratio is a continuous focus on our expense ratio. Our strategy across our segments is to maintain premium rates, deploy capital judiciously, manage our expenses and focus on the businesses in which we have expertise, which we believe should provide opportunities for greater returns. Investment income is also an important part of our business. Because the period of time between our receipt of premiums and the ultimate settlement of claims is often several years or longer, we are able to invest cash from premiums for significant periods of time. Our net investment income was $227.5 million, $208.0 million and $156.3 million for the years ended December 31, 2017, 2016 and 2015, respectively.
Product warranty registration and service - Our Specialty Risk and Extended Warranty business generates fee revenue for product warranty registration and claims handling services provided to unaffiliated third party retailers, manufacturers and dealerships. Additionally, we provide credit monitoring services for a fee. As discussed above, on February 28, 2018, we completed the transfer of the U.S.-based fee business to Mayfield and the Investor's acquisition of a majority interest in Mayfield, which we expect will impact the service and fee income generated from this source.
Loss and Loss Adjustment Expense Reserve and Unearned Premiums. We record reserves for estimated losses under insurance policies that we write and for loss adjustment expenses related to the investigation and settlement of such losses. Our reserves for loss and loss adjustment expenses represent the estimated amount to settle all reported and unreported loss and loss adjustment expenses unpaid at any given point in time based on known facts and circumstances. In establishing our reserves, we do not use loss discounting, which would involve recognizing the time value of money and offsetting estimates of future payments by future expected investment income. Our reserves for loss and loss adjustment expenses include both case-by-case valuations and actuarial estimates of potential changes to such case valuations as well as estimates of losses which have not yet been reported to us.
Net Earned Premiums. Net earned premiums increased $646.7 million, or 16.1%, to $4,668.0 million from $4,021.2 million for the years ended December 31, 2016 and 2015, respectively. The increase by segment was: Small Commercial Business - $316.6 million; Specialty Risk and Extended Warranty - $238.9 million; and Specialty Program - $91.3 million. The increase in net earned premiums resulted from an increase in gross written premiums for the year ended December 31, 2016 compared to the year ended December 31, 2015.
Net Earned Premiums Less Expenses Included in Combined Ratio (Underwriting Income). Our underwriting income decreased to $150.4 million for the year ended December 31, 2016 compared to $166.0 million for the year ended December 31, 2015. The decrease in underwriting income related primarily to an increase in combined ratio during the year ended December 31, 2016 compared to the year ended December 31, 2015, which was partially offset by an increase in the level of earned premiums.
Net Earned Premiums. Net earned premiums increased $465.9 million, or 30.2%, to $2,009.8 million for the year ended December 31, 2017 from $1,543.9 million for the year ended December 31, 2016. Net earned premiums increased due to an increase in net written premiums during the year ended December 31, 2017 compared to the same period in 2016. As net written premiums are earned ratably over the term of a policy, the increase in net earned premiums resulted from the increase in net written premiums period over period, and from policies assumed from the ANV acquisition, which had accelerated earnings patterns relative to our business prior to the acquisition.
For further information on our OTTI policy, see Note 2. “Significant Accounting Policies” to our consolidated financial statements appearing elsewhere in this Form 10-K.
There are 2,113 securities at December 31, 2017 that account for the gross unrealized loss, none of which we deem to be OTTI. Significant factors influencing our determination that unrealized losses were temporary included the magnitude of the unrealized losses in relation to each security’s cost, the nature of the investment and management’s intent not to sell these securities and it being more likely than not that we will not be required to sell these investments before anticipated recovery of fair value to our cost basis.
We earn a significant amount of our operating funds outside the U.S., which are deemed to be indefinitely reinvested in foreign jurisdictions. We currently do not intend or foresee a need to repatriate these funds. We expect existing domestic cash, short-term investments and cash flows from operations to continue to be sufficient to fund our domestic operating activities and cash commitments for investing and financing activities, such as payment of dividends, debt repayment and capital expenditures. If we should require more capital in the U.S. than is generated by our domestic operations (e.g., to fund significant discretionary activities such as business acquisitions or share repurchases), we could elect to repatriate future earnings from foreign jurisdictions.
Net cash provided by financing activities was $268.4 million for the year ended December 31, 2016 compared to $766.5 million for the year ended December 31, 2015. The decrease in cash provided by financing activities was primarily attributable to decreases in issuances of common stock and preferred stock and cash received in the year ended December 31, 2015 for the issuance of subordinated debt. The decrease in cash provided by financing activities was partially offset by increases in dividend payments to common and preferred stockholders, cash received in the year ended December 31, 2016 from entering into repurchase agreements, and cash paid in the year ended December 31, 2015 for the settlement of convertible debt.
In our proportional reinsurance programs, we generally receive a commission on the premium ceded to reinsurers. This “ceding commission” compensates our insurance companies for the direct costs associated with production of the business, the servicing of the business during the term of the policies ceded, and the costs associated with placement of reinsurance that benefits the proportional programs. In addition, certain of our reinsurance treaties allow us to share in any net profits generated under such treaties with the reinsurers. Various reinsurance brokers may arrange for the placement of this reinsurance coverage on our behalf and are compensated, directly or indirectly, by the reinsurers. We also place reinsurance with direct reinsurance markets and enter reinsurance relationships with third-party captives formed by agents and other business partners as a mechanism for sharing risk and profit. In order to reduce our exposure to reinsurance credit risk, we evaluate the financial condition of our reinsurers and place our reinsurance with a diverse group of companies and syndicates that we believe to be financially sound. We carefully monitor the credit quality of our reinsurers when we place new and renewal reinsurance, as well as on an ongoing, current basis. We use objective criteria to select and retain our reinsurers, including requiring them to be fully collateralized, maintain minimum surplus of $500.0 million or have a financial strength rating of “A-” or better from A.M. Best or Standard & Poor's Corporation. We approve exceptions to these criteria when warranted.
The following table summarizes the top ten reinsurers that account for approximately 68% of our reinsurance recoverables on paid and unpaid losses and loss adjustment expenses as of December 31, 2017:
We are subject to various annual assessments imposed by certain of the states in which we write insurance policies. These assessments are generally based upon the amount of premiums written or losses paid during the applicable year. Assessments based on premiums are generally paid within one year after the calendar year in which the policies are written, while assessments based on losses are generally paid within one year after the loss is paid. When we establish a reserve for loss and loss adjustment expenses for a reported claim, we accrue