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Stock-based Compensation (Tables)
12 Months Ended
Dec. 31, 2021
Share-based Payment Arrangement, Noncash Expense [Abstract]  
Summary of Restricted Stock Awards Activity
The following is a summary of the RSU activity during the year ended December 31, 2021.
 RSUs
 (In millions)
Outstanding at the beginning of year
Granted
Vested(1)
Forfeited— 
Outstanding at end of year
Summary of the Assumptions Used in the Monte Carlo Simulation to Determine the Grant-Date Fair Value
The following table summarizes the assumptions used in the Monte Carlo simulation to determine the grant-date fair value for the RSUs with market conditions granted during the years ended December 31, 2021, 2020 and 2019. 
 Years Ended December 31,
 202120202019
Risk-free rate0.2 %1.4 %2.5 %
Expected volatility30 %19 %18 %
Expected dividend rate3.4 %3.5 %4.0 %
Summary of Restricted Stock Vested
The following table is a summary of the RSUs vested during the years ended December 31, 2021, 2020 and 2019.
Years Ended December 31,Total Shares
Vested
Fair Value on
Vesting Date
 (In millions
of shares)
 
2021$199 
2020220 
2019135 
Stock Based Compensation Expense
The following table discloses the components of stock-based compensation expense.
 Years Ended December 31,
 202120202019
Stock-based compensation expense:
Site rental costs of operations$14 $16 $19 
Services and other costs of operations
Selling, general and administrative expenses109 109 90 
Total stock-based compensation$131 $133 $116