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Stockholders' Equity (Summary of Assumptions Used to Value Stock Option Grants) (Details)
$ / shares in Units, $ in Thousands
12 Months Ended
Dec. 31, 2017
USD ($)
$ / shares
Dec. 31, 2016
USD ($)
$ / shares
Dec. 31, 2015
USD ($)
$ / shares
Stockholders' Equity Note [Abstract]      
Dividend yield 0.00% 0.00% 0.00%
Expected volatility, minimum 34.00% 40.00% 36.00%
Expected volatility, maximum 37.00% 50.00% 53.00%
Risk-free interest rate, minimum 2.24% 1.57% 2.03%
Risk-free interest rate, maximum 2.45% 2.04% 2.29%
Suboptimal exercise factor, minimum 2.48 2.48 2.47
Suboptimal exercise factor, maximum 2.63 2.48 2.48
Valuation data:      
Weighted-average fair value (in USD per share) | $ / shares $ 71.45 $ 48.85 $ 39.22
Total stock-based compensation expense (in thousands) $ 182,209 $ 173,675 $ 124,725
Total income tax impact on provision (in thousands) $ 61,842 $ 65,173 $ 47,125