Computation of Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands		3 Months Ended								12 Months Ended
		Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Earnings Per Share [Abstract]
Net income		$ 95,060	$ 101,346	$ 119,722	$ 107,095	$ 80,469	$ 91,515	$ 104,882	$ 104,653	$ 423,223	$ 381,519	$ 311,623
Denominator for basic earnings per share—Weighted-average shares outstanding										304,722	302,343	299,574
Effect of dilutive securities:
RSUs										667	791	845
Stock options										2,107	2,905	3,696
Restricted stock										0	6	16
Denominator for diluted earnings per share—Adjusted weighted-average shares outstanding										307,496	306,045	304,131
Earnings per share, basic	[1]									$ 1.39	$ 1.26	$ 1.04
Earnings per share, diluted	[1]									$ 1.38	$ 1.25	$ 1.02

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