Earnings Per Share - Antidilutive Securities Excluded from Computation of Earnings Per Share (Details) (USD $) In Millions, except Per Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income (Loss) Available to Common Stockholders, Basic	$ 457	$ 929	$ 473	$ 858
Net Income (Loss) Available to Common Stockholders, Diluted	500	972		944
Weighted Average Number of Shares Outstanding, Diluted	760	751	653	752
Weighted Average Number of Shares Outstanding, Basic	653	642	653	642
Earnings Per Share, Basic	$ 0.70	$ 1.45	$ 0.72	$ 1.34
Earnings Per Share, Diluted	$ 0.66	$ 1.29	$ 0.72	$ 1.25
4.50% Cumulative Comvertible Preferred Stock [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income Adjustments			6
Dilutive Securities, Effect on Basic Earnings Per Share	3	3		6
Weighted Average Number Diluted Shares Outstanding Adjustment	6	6		6
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			6
5.75% Cumulative Convertible Preferred Stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income Adjustments			43
Dilutive Securities, Effect on Basic Earnings Per Share	21	21		43
Weighted Average Number Diluted Shares Outstanding Adjustment	56	56		56
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			56
5.75% Cumulative Convertible Preferred Stock Series A
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income Adjustments			32
Dilutive Securities, Effect on Basic Earnings Per Share	16	16		32
Weighted Average Number Diluted Shares Outstanding Adjustment	40	39		39
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			39
5.0% Cumulative Convertible Preferred Stock Series Two Thousand And Five B [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Net Income Adjustments			5
Dilutive Securities, Effect on Basic Earnings Per Share	3	3		5
Weighted Average Number Diluted Shares Outstanding Adjustment	5	5		5
Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount			5
Restricted stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Dilutive Securities, Effect on Basic Earnings Per Share		$ 0		$ 0
Weighted Average Number Diluted Shares Outstanding Adjustment		3		4