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Stock-Based Compensation (Tables)
 6 Months Ended
Jun. 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 six months ended June 30, 2020 (in thousands): 
 Three Months Ended
June 30,		Six Months Ended
June 30,
 2020201920202019
Stock options$5,755  $9,698  $12,774  $19,863  
Restricted stock3,235  8,608  10,390  10,984  
Stock-based compensation$8,990  $18,306  $23,164  $30,847  
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 June 30, 2020 (cost in thousands):
Unrecognized
Compensation
Cost		Weighted Average
Period of Expense
Recognition
(in Years)
Stock options$48,232  1.99
Restricted stock23,567  2.44
Total$71,799  
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 six months ended June 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  
Granted486  213.12  $52.30  
Exercised(897) 107.74  128,906  
Forfeited(70) 165.91  
Outstanding at June 30, 20205,782  $133.95  4,371  $112.88  $679,823  
Expected to vest as of June 30, 20201,411  $199.19  
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 six months ended June 30, 2020 and 2019:
 June 30,
 20202019
Risk-free interest rate0.38 %2.42 %
Dividend yield—  —  
Expected volatility30.88 %26.40 %
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 six 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 six months ended June 30, 2020 (shares in thousands): 
SharesWeighted
Average
Grant Date
Fair Value
Outstanding at December 31, 2019243  $246.34  
Granted151  218.02  
Vested(128) 222.49  
Canceled or forfeited(12) 251.49  
Outstanding at June 30, 2020254  $240.08