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Stockholders' Equity (Summary of Assumptions Used to Value Stock Option Grants) (Details) - USD ($)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Stockholders' Equity Note [Abstract]      
Dividend yield 0.00% 0.00% 0.00%
Expected volatility, minimum 37.00% 37.00% 40.00%
Expected volatility, maximum 45.00% 41.00% 42.00%
Risk-free interest rate, minimum 0.67% 1.74% 2.61%
Risk-free interest rate, maximum 1.71% 2.74% 3.09%
Suboptimal exercise factor, minimum 3.34 3.07 2.80
Suboptimal exercise factor, maximum 3.67 3.23 3.01
Valuation data:      
Weighted-average fair value (in dollars per share) $ 217.42 $ 156.60 $ 157.19
Total stock-based compensation expense $ 415,180 $ 405,376 $ 320,657
Total income tax impact on provision $ 91,718 $ 90,856 $ 67,575