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, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Basic earnings per share:
Net income	$ 26,335	$ 71,018	$ 50,031	$ 124,133
Weighted-average common shares outstanding	425,340	419,974	424,486	419,349
Basic earnings per share (in dollars per share)	$ 0.06	$ 0.17	$ 0.12	$ 0.30
Diluted earnings per share:
Net income	$ 26,335	$ 71,018	$ 50,031	$ 124,133
Shares used in computation:
Weighted-average common shares outstanding	425,340	419,974	424,486	419,349
Employee stock options	10,757	11,467	10,472	11,792
Weighted-average number of shares	436,097	431,441	434,958	431,141
Diluted earnings per share (in dollars per share)	$ 0.06	$ 0.16	$ 0.12	$ 0.29