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Stockholders' Equity (Tables)
3 Months Ended
Mar. 31, 2018
Stockholders' Equity and Stock-based Compensation Disclosure [Abstract]  
Summary of Activity Related to Stock Option Plans
A summary of the activities related to the Company’s stock option plans is as follows:
 
 
 
Options Outstanding
 
 
 
 
 
Shares
Available
for Grant
 
Number of
Shares
 
Weighted-
Average
Exercise Price
(per share)
 
Weighted-Average Remaining
Contractual Term
(in years)
 
Aggregate
Intrinsic Value
(in thousands)
Balances as of December 31, 2017
10,739,915

 
21,647,350

 
$
61.13

 
 
 
 
Granted
(553,220
)
 
553,220

 
252.69

 
 
 
 
Exercised

 
(1,264,617
)
 
43.84

 
 
 
 
Balances as of March 31, 2018
10,186,695

 
20,935,953

 
$
67.24

 
5.91
 
$
4,775,786

Vested and exercisable as of March 31, 2018
 
 
20,935,953

 
$
67.24

 
5.91
 
$
4,775,786

Summary of Amounts Related to Option Exercises
A summary of the amounts related to option exercises, is as follows:
 
Three Months Ended
 
March 31,
2018
 
March 31,
2017
 
(in thousands)
Total intrinsic value of options exercised
$
277,910

 
$
107,097

Cash received from options exercised
56,335

 
24,178

Summary of Assumptions Used to Value Stock Option Grants Using Lattice-Binomial Model
The following table summarizes the assumptions used to value option grants using the lattice-binomial model and the valuation data:
 
Three Months Ended
 
March 31,
2018
 
March 31,
2017
Dividend yield
%
 
%
Expected volatility
40
%
 
37
%
Risk-free interest rate
2.61
%
 
2.45
%
Suboptimal exercise factor
2.80

 
2.48

Weighted-average fair value (per share)
$
123.63

 
$
62.36

Total stock-based compensation expense (in thousands)
$
68,395

 
$
44,888

Total income tax impact on provision (in thousands)
$
14,691

 
$
14,701