Earnings Per Share - Computation of Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Basic earnings per share:
Net income	$ 40,755	$ 26,335	$ 68,413	$ 50,031
Weighted-average common shares outstanding (in shares)	428,483	425,340	428,300	424,486
Basic earnings per share (in dollars per share)	$ 0.10	$ 0.06	$ 0.16	$ 0.12
Diluted earnings per share:
Net income	$ 40,755	$ 26,335	$ 68,413	$ 50,031
Shares used in computation:
Weighted-average common shares outstanding (in shares)	428,483	425,340	428,300	424,486
Employee stock options (in shares)	9,671	10,757	9,773	10,472
Weighted-average number of shares (in shares)	438,154	436,097	438,073	434,958
Diluted earnings per share (in dollars per share)	$ 0.09	$ 0.06	$ 0.16	$ 0.12