$21.5 million of adverse development was experienced in 2017 on the reserve for losses and loss adjustment expenses held at December 31, 2016. This adverse reserve development included $20.0 million of adverse development in the Excess and Surplus Lines segment, including $38.7 million of adverse development in the commercial auto line of business that was primarily related to the 2016 contract year with one insured. The adverse development for commercial auto was partially offset by favorable development of $18.6 million in other Excess and Surplus Lines underwriting divisions primarily from the 2014 through 2016 accident years. This favorable development occurred because our actuarial studies at December 31, 2017 for the Excess and Surplus Lines segment indicated that our loss experience on our casualty business excluding commercial auto continued to be below our initial expected ultimate loss ratios. The Company also experienced $2.7 million of favorable development on prior accident years in the Specialty Admitted Insurance segment primarily from accident years 2010 through 2015, as losses on our workers’ compensation business written prior to 2016 continued to develop more favorably than we had anticipated. The Casualty Reinsurance segment experienced $4.2 million of adverse development on prior accident years primarily from two contracts from 2010 through 2013 that had higher than expected reported losses in 2017.
significant scale and produces a lower expense ratio than our other operating segments. The Excess and Surplus Lines segment is our largest segment and makes up 68.1% of consolidated net earned premiums for the year ended December 31, 2018 (62.5% for the year ended December 31, 2017). Gross fee income for the Company increased from $28.3 million for the year ended December 31, 2017 to $28.7 million for the year ended December 31, 2018.
Insurance premiums are earned ratably over the terms of our insurance policies, generally twelve months. Reinsurance premiums assumed are earned over the terms of the underlying policies or reinsurance contracts. Contracts and policies written on a “losses occurring” basis cover claims that may occur during the term of the contract or insurance policy, which is typically twelve months. Reinsurance contracts which are written on a “risks attaching” basis cover claims which attach to the underlying insurance policies written during the terms of such contracts. Premiums earned on such contracts usually extend beyond the original term of the reinsurance contract, typically resulting in recognition of premiums earned over a 24-month period in proportion to the level of underlying exposure.
2017, respectively, while the net retention on the workers’ compensation business was 45.9% and 64.8%, respectively. The significant decline in the net retention from the prior year on the workers' compensation business is due to the 50% third-party quota share reinsurance coverage purchased in this business effective October 1, 2017.
The expense ratio of the Casualty Reinsurance segment was 34.1% for the year ended December 31, 2018 compared to the prior year ratio of 34.5% . The expense ratio for the year ended December 31, 2017 was impacted by the correction of a contingent commission accrual and related commission expense, which increased underwriting expenses by $2.0 million in the year ended December 31, 2017.
The Company’s private investments, including our renewable energy LLC investments, produced positive returns in 2018, but fell short of the strong prior year results. The decrease in private investments was offset by increased income from our growing fixed income portfolio of bonds and bank loan participations which benefited from rising investment yields in 2018.
Of our total cash and invested assets of $1,850.3 million at December 31, 2018, $172.5 million represents the cash and cash equivalents portion of the portfolio. The majority of the portfolio, or $1,184.2 million, is comprised of fixed maturity securities that are classified as available-for-sale and carried at fair value with unrealized gains and losses on these securities reported, net of applicable taxes, as a separate component of accumulated comprehensive income or loss. Also included in our investments are $261.0 million of bank loan participations, $78.4 million of equity securities, $82.0 million of short-term investments, and $72.3 million of other invested assets.
Corporate available-for-sale fixed maturity securities include public traded securities and privately placed bonds as shown below as of December 31, 2018:
Our insurance segments enter into reinsurance contracts to limit our exposure to potential losses arising from large risks, to protect against the aggregation of several risks in a common loss occurrence, and to provide additional capacity for growth. Our reinsurance is contracted under excess of loss and quota share reinsurance contracts. In excess of loss reinsurance, the reinsurer agrees to assume all or a portion of the ceding company’s losses in excess of a specified amount. The premiums payable to the reinsurer are negotiated by the parties based on their assessment of the amount of risk being ceded to the reinsurer because the reinsurer does not share proportionately in the ceding company’s losses. In quota share reinsurance, the reinsurer agrees to assume a specified percentage of the ceding company’s losses arising out of a defined class of business in exchange for a corresponding percentage of premiums. For the years ended December 31, 2018, 2017, and 2016 our net premium retention was 65.4%, 70.9% and 75.6%, respectively.