The weighted average assumptions used in the measurement of our net periodic benefit costs for the years ended December 31 are as follows: During 2020, the fair value of plan assets in our U.S. qualified defined benefit pension plan increased $110.9 million, or 6.9 percent due to a favorable return on assets which resulted in a gain of approximately 14.2 percent, partially offset by the payment of benefits and expenses. The fair value of plan assets in our U.K. pension plan increased £24.4 million, or 12.8 percent, due primarily to a favorable return on assets which resulted in a gain of approximately 14.9 percent. Although our rate of return on plan assets for 2020 exceeded our assumptions used in the measurement of our net periodic benefit costs, we believe our assumptions appropriately reflect the impact of the current economic environment and our expectations for the future investment returns based on the plan's asset allocation. As of December 31, 2020, our pension and OPEB plans have an aggregate unrecognized net actuarial loss of $827.4 million and an unrecognized prior service credit of $2.1 million, which together represent the cumulative liability and asset gains and losses as well as the portion of prior service credits that have not been recognized in pension expense. The unrecognized net actuarial loss for our pension plans, which is $838.4 million at December 31, 2020, will be amortized over the average remaining life expectancy of the plan, which is approximately 25  years for the U.S. plan and 31 years for the U.K. plan, to the extent that it exceeds the 10 percent corridor, as described below. The unreco gnized net actuarial gain of $11.0 million for our OPEB plan will be amortized over the average future working life of OPEB plan participants, estimated at three  years, to the extent the gain is outside of the corridor. The corridor for the pension and OPEB plans is established based on the greater of 10 percent of the plan assets or 10 percent of the benefit obligation. At December 31, 2020, $539.4 million of the actuarial loss was outside of the corridor for the U.S. plans and £29.6 million was outside of the corridor for the U.K. plan. At December 31, 2020, no ne of  the actuarial gain was outside of the corridor for the OPEB plan. The amortization of the unrecognized actuarial gain or loss and the unrecognized prior service credit is a component of our net periodic benefit cost and equaled $19.7 million, $18.4 million, and $22.1 million in 2020, 2019, and 2018, respectively. The fair value of plan assets in our U.S. qualified defined benefit pension plan was $1,710.9 million at December 31, 2020, compared to $1,600.0 million at December 31, 2019. The plan was in an underfunded position of $339.0 million and $300.8 million at December 31, 2020 and December 31, 2019, respectively. This year-over-year change was due primarily to the increase in period benefit obligations due to the decrease in discount rate, partially offset by higher than expected asset returns. The fair value of plan assets in our U.K. pension plan was £215.1 million at December 31, 2020, compared to £190.7 million at December 31, 2019. The U.K. pension plan was in an underfunded position of £4.3 million and £3.1 million at December 31, 2020 and 2019, respectively. This year-over- year change was due primarily to the increase in period benefit obligations due to the decrease in discount rate, partially offset by higher than expected asset returns. 57 Benefit Obligation and Fair Value of Plan Assets The following illustrates the sensitivity of the below items to a 50 basis point change in the discount rate or the expected long-term rate of return on plan assets: ($ in millions) At or for the Year Ended December 31, 2020 Assumption Change  Net Periodic Benefit Cost, Before Tax Benefit Obligation Stockholders' Equity, After Tax Discount Rate + 50 bp $ (2.3) $ (201.8) $ 160.1  Discount Rate - 50 bp 0.4  226.4  (179.5) Expected Long-term Rate of Return on Plan Assets + 50 bp (8.8) N/A N/A Expected Long-term Rate of Return on Plan Assets - 50 bp 8.8  N/A N/A Pension Benefits U.S. Plans U.K. Plan OPEB Assumption 2021 2020 2021 2020 2021 2020 Discount Rate 2.90  % 3.60  % 1.40  % 2.00  % 2.60  % 3.40  % Expected Long-term Rate of Return on Plan Assets 6.00  % 7.00  % 3.50 % 4.10 % 5.75  % 5.75  %