Earnings Per Share - Computation of Net Income Per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Basic earnings per share:
Net income	$ 29,432	$ 59,295	$ 79,463	$ 183,428
Weighted-average common shares outstanding (in shares)	426,869	421,194	425,289	419,972
Basic earnings per share (in dollars per share)	$ 0.07	$ 0.14	$ 0.19	$ 0.44
Diluted earnings per share:
Net income	$ 29,432	$ 59,295	$ 79,463	$ 183,428
Shares used in computation:
Weighted-average common shares outstanding (in shares)	426,869	421,194	425,289	419,972
Employee stock options (in shares)	10,737	11,548	10,560	11,711
Weighted-average number of shares (in shares)	437,606	432,742	435,849	431,683
Diluted earnings per share (in dollars per share)	$ 0.07	$ 0.14	$ 0.18	$ 0.42