Earnings per Share (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015	Dec. 31, 2015
Basic earnings per share [Abstract]
Net income	$ 56,618	$ 822,852	$ 235,030	$ 1,069,582
Weighted average common shares outstanding (in shares)	349,376	339,701	343,403	339,504
Basic income per share (in dollars per share)	$ 0.16	$ 2.42	$ 0.68	$ 3.15
Diluted earnings per share [Abstract]
Net income	$ 56,618	$ 822,852	$ 235,030	$ 1,069,582
Diluted income available to common shareholders	$ 58,615	$ 833,581	$ 245,398	$ 1,101,768
Weighted-average shares - Basic (in shares)	349,376	339,701	343,403	339,504
Effect of dilutive securities [Abstract]
Weighted-average shares - Diluted (in shares)	406,050	468,128	421,423	468,076
Diluted income per share (in dollars per share)	$ 0.14	$ 1.78	$ 0.58	$ 2.35
Antidilutive securities (in shares)	19,000	0	20,400	0
Effective income tax rate (in hundredths)	35.00%	35.00%	35.00%	35.00%
2% Convertible Senior Notes due 2020
Diluted earnings per share [Abstract]
Dilutive securities	$ 1,324	$ 1,982	$ 5,288	$ 5,946
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)	44,488	71,917	62,707	71,917
Stated interest rate (in hundredths)	2.00%	2.00%	2.00%	2.00%	2.00%
5% Convertible Senior Notes due 2017
Diluted earnings per share [Abstract]
Dilutive securities	$ 673	$ 3,050	$ 5,080	$ 9,150
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)	10,791	25,674	13,885	25,674
Stated interest rate (in hundredths)	5.00%	5.00%	5.00%	5.00%	5.00%
9% Convertible Junior Subordinated Debentures due 2063
Diluted earnings per share [Abstract]
Dilutive securities	$ 0	$ 5,697	$ 0	$ 17,090
Effect of dilutive securities [Abstract]
Dilutive securities (in shares)	0	28,853	0	28,853
Stated interest rate (in hundredths)	9.00%	9.00%	9.00%	9.00%
Unvested Restricted Stock Units
Effect of dilutive securities [Abstract]
Dilutive securities - Share based compensation (in shares)	1,395	1,983	1,428	2,128