Favorable prior year development (net of additional and return premiums) was $37 million in 2017. Favorable prior year development (net of additional and return premiums) was $59 million in 2016. 90 Insurance - Reserves for the Insurance segment developed favorably by $68 million in 2017. The favorable development was primarily attributable to workers' compensation business, and was partially offset by unfavorable development for professional liability business. For workers' compensation, the favorable development was related to both primary and excess business and was spread across many accident years, including those prior to 2008, but was most significant in accident years 2014 through 2016. The favorable workers' compensation development reflects a continuation during 2017 of the generally benign loss cost trends experienced in recent years, particularly the favorable claim frequency trends (i.e. number of reported claims per unit of exposure). Reported workers' compensation losses in 2017 continued to be better than our expectations at most of our operating units, and were below the assumptions underlying our previous reserve estimates. The favorable severity trends were also impacted by our continued investment in medical case management services and the higher usage of preferred provider networks. The long term trend of declining workers' compensation frequency can be attributed to improved workplace safety. For professional liability business, adverse development was primarily related to unexpected large D&O liability losses at one of our U.S. operating units, and large professional indemnity and D&O losses in the U.K. The adverse development stemmed mainly from accident years 2013 through 2016 in the U.S. and 2011 through 2016 in the U.K. Reinsurance - Reserves for the Reinsurance segment developed unfavorably by $31 million in 2017. This adverse development was due to reserve strengthening associated with claims impacted by the change in the Ogden discount rate in the U.K., as well as adverse development on the U.S. facultative casualty excess of loss business. The Ogden rate is the discount rate used to calculate lump-sum bodily injury payouts in the U.K., and was reduced by the U.K. Ministry of Justice from +2.5% to -0.75% in 2017; the adverse development mostly related to U.K. motor bodily injury claims which we reinsured on an excess of loss basis in accident years 2012 through 2016. The adverse development on U.S. facultative casualty business was due to construction related risks in accident years 2008 and prior. Insurance - Reserves for the Insurance segment developed favorably by $53 million in 2016. The favorable development was primarily related to workers' compensation business, and was partially offset by unfavorable development for medical professional liability business. For workers' compensation, the favorable development was related to both primary and excess business and to many accident years, including those prior to 2007. During 2016, reported workers' compensation losses continued to be better than our expectations at most of our operating units. Loss frequency and severity trends continued to be better than the assumptions underlying our previous reserve estimates. Loss severity trends also benefited from our continued investment in medical case management services and from our preferred provider networks. The long term trend of declining workers' compensation frequency can be attributed to improved workplace safety. For medical professional liability business, unfavorable development was primarily related to a class of business that has been discontinued. The adverse development for that business stemmed mainly from accident years 2010 through 2015. Reinsurance - Reserves for the Reinsurance segment developed favorably by $6 million in 2016. The favorable development was primarily related to direct facultative reinsurance business and to accident years 2008 through 2014. Environmental and Asbestos — To date, known environmental and asbestos claims have not had a material impact on the Company’s operations, because its subsidiaries generally did not insure large industrial companies that are subject to significant environmental or asbestos exposures prior to 1986 when an absolute exclusion was incorporated into standard policy language. The Company’s net reserves for losses and loss expenses relating to asbestos and environmental claims on policies written before adoption of the absolute exclusion was $28 million at December 31, 2018 and $30 million at December 31, 2017. The estimation of these liabilities is subject to significantly greater than normal variation and uncertainty because it is difficult to make an actuarial estimate of these liabilities due to the absence of a generally accepted actuarial methodology for these exposures and the potential effect of significant unresolved legal matters, including coverage issues, as well as the cost of litigating the legal issues. Additionally, the determination of ultimate damages and the final allocation of such damages to financially responsible parties are highly uncertain. Discounting — The Company discounts its liabilities for certain workers’ compensation reserves. The amount of workers’ compensation reserves that were discounted was $1,793 million and $1,855 million at December 31, 2018 and 2017, respectively. The aggregate net discount for those reserves, after reflecting the effects of ceded reinsurance, was $563 million and $591 million at