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Employee Stock Plans (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Disclosure of Compensation Related Costs, Share-based Payments [Abstract]      
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Authorized 19,100,000    
Share-based Compensation Arrangement by Share-based Payment Award, Additional Registered Shares 5,600,000    
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant 4,896,220 5,862,454  
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Total Intrinsic Value $ 15,320 $ 10,511 $ 10,400
Proceeds from exercise of stock options 22,195 18,967 14,900
Amortization of stock-based compensation 39,122 39,825 47,274
Tax benefits related to stock-based compensation awards $ 18,846 $ 19,745 $ 16,593
Share-based Compensation Arrangement by Share-based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value $ 20.01 $ 16.93 $ 19.10
Non-employee director award $ 130