Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Income used in computing basic and diluted earnings per share	$ 531,451	$ 521,759	$ 692,164
Denominator:
Denominator for basic earnings per share-weighted-average common shares (in shares)	165,672	171,995	179,676
Effect of dilutive securities – employee stock compensation plan (in shares)	832	937	859
Denominator for diluted earnings per share- adjusted weighted-average shares outstanding (in shares)	166,504	172,932	180,535
Basic (in dollars per share)	$ 3.21	$ 3.03	$ 3.85
Diluted (in dollars per share)	$ 3.19	$ 3.02	$ 3.83