Our U.S. defined benefit plan assets consist of a balanced portfolio of equity and fixed income securities. Our non-U.S. defined benefit plan assets include a significant concentration of United Kingdom ("U.K.") fixed income securities. We monitor investment allocations and manage plan assets to maintain acceptable levels of risk. For all periods presented, we used a measurement date of December 31 for each of our U.S. pension plans, non-U.S. pension plans and postretirement medical plans. We maintain qualified and non-qualified defined benefit pension plans in the U.S. The qualified plan provides coverage for substantially all full- time U.S. employees who receive benefits, up to an earnings threshold specified by the U.S. Department of Labor. The non-qualified plans primarily cover a small number of employees including current and former members of senior management, providing them with benefit levels equivalent to other participants, but that are otherwise limited by U.S. Department of Labor rules. The U.S. plans are designed to operate as "cash balance" arrangements, under which the employee has the option to take a lump sum payment at the end of their service. The difference between total accumulated benefit obligation and total projected benefit obligation ("Benefit Obligation") is immaterial. At December 31, 2020 as compared with December 31, 2019, we decreased our discount rate from 3.41% to 2.62% based on an analysis of publicly-traded investment grade U.S. corporate bonds, which had a lower yield due to current market conditions. In determining 2020 expense, the expected rate of return on U.S. plan assets remained constant at 6.0 0%, primarily based on our target allocations and expected long-term asset returns. The long-term rate of return assumption is calculated using a quantitative approach that utilizes unadjusted historical returns and asset allocation as inputs for the calculation. F or all U.S. plans, we adopted the Pri-2012 mortality tables and the MP-2020 improvement scale published in October 2020. We applied the Pri-2012 tables based on the constituency of our plan population for union and non-union participants. We adjusted the improvement scale to utilize the Proxy SSA Long Term Improvement Rates ("LTIR"), consistent with assumptions adopted by the Social Security Administration trustees, based on long-term historical experience. Currently, we believe this approach provides the best estimate of our future obligation. Most plan participants elect to receive plan benefits as a lump sum at the end of service, rather than an annuity. As such, the updated mortality tables had an immaterial effect on our pension obligation. U.S. Defined Benefit Plans  Year Ended December 31, 2020 2019 2018 Weighted average assumptions used to determine Benefit Obligations: Discount rate 2.62  % 3.41  % 4.34  % Rate of increase in compensation levels 3.63  3.50  3.50  Weighted average assumptions used to determine net pension expense: Long-term rate of return on assets 6.00  % 6.00  % 6.00  % Discount rate 3.41  4.34  3.63  Rate of increase in compensation levels 3.56  3.50  4.01  Weighted-average interest crediting rates 3.79  % 3.79  % 3.79  % The following are assumptions related to the U.S. defined benefit pension plans: 92