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Earnings Per Share (Details) - USD ($)
$ / shares in Units, shares in Thousands, $ in Thousands
3 Months Ended 9 Months Ended
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Sep. 30, 2019
Sep. 30, 2018
Earnings Per Share [Abstract]                
Net Income $ 79,525 $ 131,390 $ 32,637 $ 69,261 $ 117,049 $ 31,339 $ 243,551 $ 217,649
Weighted average shares outstanding: [Abstract]                
Basic (in shares) 39,933     40,422     39,750 40,416
Effect of dilutive securities: [Abstract]                
Stock options and employee stock purchase plan (in shares) 932     1,375     1,061 1,415
Diluted (in shares) 40,865     41,797     40,811 41,831
Basic (in dollars per share) $ 1.99     $ 1.71     $ 6.13 $ 5.39
Diluted (in dollars per share) $ 1.95     $ 1.66     $ 5.97 $ 5.20
Anti-dilutive stock options excluded from diluted earnings per share computations (in shares) 0     0     1 0