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Stockholders' Equity (Tables)
12 Months Ended
Dec. 31, 2020
Stockholders' Equity Note [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:
 
 Shares Available
for Grant
Options Outstanding
 Number of
Shares
Weighted- Average Exercise Price
(per Share)
Balances as of December 31, 201710,739,915 21,647,350 $61.13 
Granted(2,039,974)2,039,974 311.66 
Exercised— (3,205,911)38.66 
Expired— (2,135)4.60 
Balances as of December 31, 20188,699,941 20,479,278 $89.61 
Granted(2,588,380)2,588,380 320.66 
Exercised— (2,208,052)32.88 
Expired— (280)6.74 
Balances as of December 31, 20196,111,561 20,859,326 $124.28 
New Shares Authorized17,500,000 — — 
Granted(1,909,476)1,909,476 432.34 
Exercised— (4,088,612)58.35 
Expired— (3,380)27.54 
Balances as of December 31, 202021,702,085 18,676,810 $170.23 
Vested and exercisable at December 31, 2020
18,676,810 $170.23 
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:
 
 Year Ended December 31,
 202020192018
Dividend yield— %— %— %
Expected volatility
37% - 45%
37% - 41%
40% - 42%
Risk-free interest rate
0.67% - 1.71%
1.74% - 2.74%
2.61% - 3.09%
Suboptimal exercise factor
3.34 - 3.67
3.07 - 3.23
2.80 - 3.01
Valuation data:
Weighted-average fair value (per share)$217.42 $156.60 $157.19 
Total stock-based compensation expense (in thousands)415,180 405,376 320,657 
Total income tax impact on provision (in thousands)91,718 90,856 67,575