Earnings 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 [Abstract]
Net income	$ 113,654	$ 45,522	$ 246,730	$ 105,504
Weighted average common shares outstanding (in shares)	339,705	338,626	339,406	338,419
Basic income per share (in dollars per share)	$ 0.33	$ 0.13	$ 0.73	$ 0.31
Diluted earnings per share [Abstract]
Net income	$ 113,654	$ 45,522	$ 246,730	$ 105,504
Diluted income available to common shareholders	$ 121,395	$ 48,571	$ 262,212	$ 111,602
Weighted-average shares - Basic (in shares)	339,705	338,626	339,406	338,419
Effect of dilutive securities [Abstract]
Weighted-average shares - Diluted (in shares)	439,148	413,481	439,221	413,374
Diluted income per share (in dollars per share)	$ 0.28	$ 0.12	$ 0.60	$ 0.27
Antidilutive securities (in shares)	28,900	54,500	28,900	54,500
2% Convertible Senior Notes due 2020 [Member]
Effect of dilutive securities [Abstract]
Dilutive securities	$ 3,049	$ 3,049	$ 6,098	$ 6,098
Dilutive securities (in shares)	71,942	71,942	71,942	71,942
5% Convertible Senior Notes due 2017 [Member]
Effect of dilutive securities [Abstract]
Dilutive securities	$ 4,692	$ 0	$ 9,384	$ 0
Dilutive securities (in shares)	25,670	0	25,670	0
Restricted Stock Units [Member] | Unvested Restricted Stock Units [Member]
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)	1,831	2,913	2,203	3,013