Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Numerator:
Income used in computing basic and diluted earnings per share	$ 692,164	$ 752,207	$ 844,611
Denominator:
Denominator for basic earnings per share-weighted-average common shares (in shares)	179,676	202,681	216,305
Effect of dilutive securities – employee stock compensation plan (in shares)	859	1,005	1,401
Denominator for diluted earnings per share- adjusted weighted-average shares outstanding (in shares)	180,535	203,686	217,706
Basic (in dollars per share)	$ 3.85	$ 3.71	$ 3.90
Diluted (in dollars per share)	$ 3.83	$ 3.69	$ 3.88