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, 2017 | 10,739,915 | | | 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, 2018 | 8,699,941 | | | 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, 2019 | 6,111,561 | | | 20,859,326 | | | $ | 124.28 | | New Shares Authorized | 17,500,000 | | | — | | | — | | 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, 2020 | 21,702,085 | | | 18,676,810 | | | $ | 170.23 | | Vested and exercisable at December 31, 2020 | | | 18,676,810 | | | $ | 170.23 | |
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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, | | | 2020 | | 2019 | | 2018 | 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 | |
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