Accounting Policies - Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounting Policies [Abstract]
Net income	$ 433,887	$ 451,295	$ 419,375
Weighted average shares outstanding - basic	85,840	84,259	83,973
Dilutive effect of stock plans	1,448	1,666	1,940
Weighted average shares outstanding - diluted	87,288	85,925	85,913
Basic earnings per share	$ 5.05	$ 5.36	$ 4.99
Diluted earnings per share	$ 4.97	$ 5.25	$ 4.88
Anti-dilutive shares	23	14	7