Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Income used in computing basic and diluted earnings per share	$ 521,759	$ 692,164	$ 752,207
Denominator:
Denominator for basic earnings per share-weighted-average common shares (in shares)	171,995	179,676	202,681
Effect of dilutive securities – employee stock compensation plan (in shares)	937	859	1,005
Denominator for diluted earnings per share- adjusted weighted-average shares outstanding (in shares)	172,932	180,535	203,686
Basic (in dollars per share)	$ 3.03	$ 3.85	$ 3.71
Diluted (in dollars per share)	$ 3.02	$ 3.83	$ 3.69