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Stock-Based Compensation (Tables)
 3 Months Ended
Mar. 31, 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 months ended March 31, 2020 (in thousands): 
 Three Months Ended
March 31,
 20202019
Stock options$7,020  $10,165  
Restricted stock7,155  2,376  
Stock-based compensation$14,175  $12,541  
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 March 31, 2020 (cost in thousands):
Unrecognized
Compensation
Cost		Weighted Average
Period of Expense
Recognition
(in Years)
Stock options$41,338  1.92
Restricted stock29,070  2.08
Total$70,408  
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 three months ended March 31, 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  
Granted243  204.30  $48.65  
Exercised(539) 133.20  28,748  
Forfeited(53) 149.00  
Outstanding at March 31, 20205,914  $126.68  4,620  $107.04  $354,020  
Expected to vest as of March 31, 20201,294  $196.76  
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 three months ended March 31, 2020 and 2019:
 March 31,
 20202019
Risk-free interest rate0.41 %2.49 %
Dividend yield—  —  
Expected volatility30.29 %26.64 %
Expected life (in years)3.84.0
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 quarter 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 three months ended March 31, 2020 (shares in thousands): 
SharesWeighted
Average
Grant Date
Fair Value
Outstanding at December 31, 2019243  $246.34  
Granted71  213.90  
Vested(79) 201.09  
Canceled or forfeited(4) 251.62  
Outstanding at March 31, 2020231  $249.52