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Stock Options and Employee Stock Purchase Plan (Tables)
 9 Months Ended
Sep. 30, 2020
Share-based Payment Arrangement [Abstract]  
Stock Option Activity
The following table represents stock option activity for the nine months ended September 30, 2020:
Number of SharesWeighted-
Average
Exercise
Price		Weighted-Average
Remaining
Contractual
Term (in years)		Aggregate
Intrinsic
Value
Outstanding at January 1, 2020518,341 $65.77 
Granted— — 
Exercised(41,530)22.92 
Canceled— — 
Outstanding at September 30, 2020476,811 $69.51 6.4$2,187,385 
Exercisable at September 30, 2020176,811 $60.13 4.9$2,187,385 
Vested and expected to vest at September 30, 2020394,131 $68.35 6.2$2,187,385 
Market-Based Restricted Stock Awards, Valuation Assumptions
The weighted-average fair values of market-based restricted stock awards granted have been estimated utilizing the following assumptions:
September 30, 2020September 30, 2019
Underlying stock price at valuation date$91.17 $84.58 
Expected volatility27.0 %28.3 %
Risk-free interest rate0.7 %2.53 %
Restricted Stock and Restricted Stock Unit Award Activity
Restricted stock award activity for the nine months ended September 30, 2020 is set forth below:
SharesWeighted-Average
Grant-Date
Fair Value
Nonvested at January 1, 20201,105,059 $64.76 
Granted1,268 98.63 
Vested(261,396)70.20 
Canceled(11,460)78.88 
Nonvested at September 30, 2020833,471 $62.91 
  
Restricted stock unit award activity for the nine months ended September 30, 2020 is set forth below:
Number of
Shares		Weighted-Average
Remaining
Contractual
Term (in years)		Aggregate
Intrinsic
Value
Outstanding at January 1, 202020,874 
Granted206,886 
Vested(6,498)
Canceled(5,297)
Outstanding at September 30, 2020215,965 3.8$14,949,097 
Vested and expected to vest at September 30, 2020140,841 3.1$9,748,995 
Employee Stock Purchase Plan, Valuation Assumptions
The compensation expense related to the Purchase Plan has been estimated utilizing the following assumptions:
September 30, 2020September 30, 2019
Risk-free interest rate0.13%2.43%
Expected term (in years)0.50.5
Dividend yield0.00%1.05%
Expected volatility23.95%26.80%
Weighted average volatility23.95%26.80%