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Stock-Based Compensation (Tables)
 12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement [Abstract]  
Summary of Restricted and Deferred Stock Activity A summary of restricted and deferred stock activity follows:
 Number of shares
Non-vested shares outstanding at January 1, 20211,820,159 
Awarded:
Time-vesting221,042 
Performance-based170,036 
Released:
Time-vesting(418,426)
Performance-based(224,358)
Forfeitures:
Time-vesting(284,247)
Performance-based— 
Non-vested shares outstanding at December 31, 20211,284,206 
Summary of Average Grant-Date Fair Value of Restricted Stock
The average grant-date fair value of restricted stock awarded in 2021, 2020 and 2019 follows:

202120202019
Time-vesting$100.08 $70.07 $56.06 
Performance-based100.99 72.08 46.08 
Schedule of Weighted Average Assumptions on Performance-Based Shares The fair values of the performance-based awards that include a market condition were calculated using a Monte Carlo valuation model and the following weighted average assumptions:
202120202019
Risk-free interest rate0.2 %1.6 %2.5 %
Expected term (years)333
Expected stock price volatility35.5 %22.0 %21.4 %