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Stock-based Compensation - (Tables)
 12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Schedule of non-vested restricted common shares
The following is summary of the status of the Trust’s non-vested restricted common shares during 2021, 2020, and 2019:
 Common SharesWeighted
Average Grant
Date Fair Value
Non-vested at December 31, 2018225,139 $15.29 
Granted194,413 17.80 
Vested(200,104)15.13 
Forfeited(2,571)16.96 
Non-vested at December 31, 2019216,877 17.67 
Granted190,418 19.00 
Vested(189,642)17.81 
Forfeited(1,831)16.80 
Non-vested at December 31, 2020215,822 18.73 
Granted224,163 17.42 
Vested(185,968)18.94 
Forfeited(6,570)18.05 
Non-vested at December 31, 2021247,447 $17.41 
Schedule of weighted average grant date fair value assumptions The Company utilized a Monte Carlo simulation to calculate the weighted average grant date fair values of $29.18 in 2021, $31.95 in 2020, and $32.65 in 2019 per unit, respectively, using the following assumptions:
202120202019
Volatility33.3 %20.1 %21.8 %
Dividend assumptionreinvestedreinvestedreinvested
Expected term in years2.8 years2.8 years2.8 years
Risk-free rate0.25 %0.84 %2.53 %
Stock price (per share)$17.21 $19.30 $17.89 
Summary of the activity in the restricted share units
The following is a summary of the activity in the Trust’s restricted share units during 2021, 2020, and 2019: 
Executive AwardsTrustee Awards
 Restricted Share
Units		Weighted
Average Grant
Date Fair Value		Restricted Share
Units		Weighted
Average Grant
Date Fair Value
Non-vested at December 31, 2018533,155 $22.66 67,158 $16.01 
Granted229,884 25.27 41,925 17.89 
Vested(104,553)(1)26.33 (41,786)16.75 
Forfeited(3,734)23.08 — — 
Non-vested at December 31, 2019654,752 22.99 67,297 16.72 
Granted482,646 21.64 38,858 19.30 
Vested(173,259)(2)29.34 (46,335)16.19 
Non-vested at December 31, 2020964,139 21.17 59,820 18.81 
Granted265,275 22.00 56,925 17.35 
Vested(252,844)(3)16.58 (53,737)18.38 
Non-vested at December 31, 2021976,570 $22.59 63,008 $17.85 
(1)Restricted units vested by Company management in 2019 resulted in the issuance of 87,805 common shares, less 35,265 common shares withheld to cover minimum withholding tax obligations, for multiple employees.
(2)Restricted units vested by Company management in 2020 resulted in the issuance of 147,765 common shares, less 65,513 common shares withheld to cover minimum withholding tax obligations, for multiple employees.
(3)Restricted units vested by Company management in 2021 resulted in the issuance of 399,165 common shares, less 162,173 common shares withheld to cover minimum withholding tax obligations, for multiple employees.