Note 11 - Stock-Based Compensation - Continued NOTES TO CONSOLIDATED FINANCIAL STATEMENTS - Continued Unum Group and Subsidiaries an adjustment to reflect the application of the performance factor to the 2017 PSU grant, which occurred during the first quarter of 2020. At December 31, 2020, we had approximately $6.5 million of unrecognized compensation cost related to PSUs that will be recognized over a weighted average period of 1.6 years. The estimated compensation expense is adjusted for actual performance experience and is recognized ratably during the service period, or remaining service period, if and when it becomes probable that the performance conditions will be satisfied. Compensation cost for PSUs subject to accelerated vesting at the date of retirement eligibility is recognized over the implicit service period. The fair value of PSUs is estimated on the date of initial grant using the Monte-Carlo sim ulation m odel. Key assumptions used to value PSUs granted during the years shown are as follows: Year Ended December 31 2020 2019 2018 Expected Volatility (based on our and our peer group historical daily stock prices) 23  % 23  % 24  % Expected Life (equals the performance period) 3 years 3 years 3 years Risk Free Interest Rate (based on U.S. Treasury yields at the date of grant) 0.85  % 2.53  % 2.32  % Restricted Stock Units (RSUs) Activity for RSUs classified as equity is as follows: During 2020, 2019, and 2018, we issued RSUs with a weighted average grant date fair value per share of $22.71, $37.07, and $47.76, respectively.  RSUs vest over a one to three-year service period, beginning at the date of grant, and the compensation cost is recognized ratably during the vesting period. Forfeitable dividend equivalents on RSUs have previously been accrued in the form of additional RSUs. Beginning with the March 1, 2020 grant, forfeitable dividends are accrued as cash. Compensation cost for RSUs subject to accelerated vesting at the date of retirement eligibility is recognized over the implicit service period. The total fair value of shares vested during 2020, 2019, and 2018 was $25.1 million, $19.5 million, and $18.1 million, respectively. At December 31, 2020, we had $21.2 million of unrecognized compensation cost related to RSUs that will be recognized over a weighted average period of 0.9 years. Weighted Average Shares Grant Date (000s) Fair Value Outstanding at December 31, 2019 1,044  $ 41.06  Granted 1,262  22.71  Vested (626) 40.11  Forfeited (87) 28.75  Outstanding at December 31, 2020 1,593  27.57  Activity for cash-settled RSUs classified as a liability is as follows: Cash-Settled RSUs Weighted Average Shares Grant Date (000s) Fair Value Outstanding at December 31, 2019 —  $ —  Granted 68  22.94  Outstanding at December 31, 2020 68  22.94  193