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Stock-Based Compensation (Tables)
 9 Months Ended
Sep. 30, 2020
Share-based Payment Arrangement [Abstract]  
Summary of Expense Related to Share-Based Payments
The table below summarizes the expense related to share-based payments recognized in the three and nine months ended September 30, 2020 (in thousands): 
 Three Months Ended
September 30,		Nine Months Ended
September 30,
 2020201920202019
Stock options$5,294 $7,038 $18,069 $26,901 
Restricted stock6,610 8,235 16,999 19,219 
Stock-based compensation$11,904 $15,273 $35,069 $46,120 
Summary of Total Unrecognized Compensation Cost Related to Stock-Based Compensation
The following table summarizes the Company’s total unrecognized compensation cost related to stock-based compensation as of September 30, 2020 (cost in thousands):
Unrecognized
Compensation
Cost		Weighted Average
Period of Expense
Recognition
(in Years)
Stock options$39,576 1.90
Restricted stock33,479 2.07
Total$73,055 
Summary of Changes in Number of Shares of Common Stock Under Option
The following summarizes the changes in the number of shares of common stock under option for the nine months ended September 30, 2020 (shares/options and aggregate intrinsic value in thousands):
SharesWeighted
Average
Exercise
Price		Options
Exercisable
at End of
Period		Weighted
Average
Exercise
Price of
Exercisable
Options		Weighted
Average Fair
Value of
Options
Granted 
During the Period		Aggregate
Intrinsic
Value
Outstanding at December 31, 20196,263 $124.38 5,137 $109.03 $1,022,860 
Granted488 214.31 $52.76 
Exercised(1,240)86.52 188,010 
Forfeited(122)192.81 
Outstanding at September 30, 20205,389 $139.73 4,135 $121.04 $530,017 
Expected to vest as of September 30, 20201,253 $201.40 
Schedule of Weighted-Average Assumptions
The fair value of stock option awards granted was estimated using the Black-Scholes option pricing model with the following weighted-average assumptions for grants or modifications during the nine months ended September 30, 2020 and 2019:
 September 30,
 20202019
Risk-free interest rate0.38 %2.40 %
Dividend yield— — 
Expected volatility30.91 %26.41 %
Expected life (in years)3.93.7
The fair value of restricted stock units granted with market based vesting conditions was estimated using the Monte Carlo simulation valuation model with the following assumptions during 2019. There were no restricted stock shares granted with market based vesting conditions during the first nine months of 2020.
    2019
Risk-free interest rate1.48%
Dividend yield—
Expected volatility25.40%
Expected life (in years)2.36
Summary of Changes in Number of Shares of Restricted Stock and Restricted Stock Units
The following table summarizes the changes in the number of shares of restricted stock and restricted stock units for the nine months ended September 30, 2020 (shares in thousands): 
SharesWeighted
Average
Grant Date
Fair Value
Outstanding at December 31, 2019243 $246.34 
Granted171 259.35 
Vested(130)223.48 
Canceled or forfeited(97)256.92 
Outstanding at September 30, 2020187 $261.94