Equity - Preferred Stock Table (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred Stock, Shares Outstanding, Beginning of Period			5,839,506
Preferred Stock, Shares Outstanding, End of Period	5,603,458		5,603,458
Loss on Exchange of Preferred Stock	$ 0	$ 0	$ (41)	$ 0
5.75% Cumulative Convertible Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred Stock, Dividend Rate, Percentage			5.75%
5.75% Cumulative Convertible Preferred Stock Series A [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred Stock, Dividend Rate, Percentage			5.75%
4.50% Cumulative Convertible Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred Stock, Dividend Rate, Percentage			4.50%
5.0% Cumulative Convertible Preferred Stock Series 2005 B [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred Stock, Dividend Rate, Percentage			5.00%
Preferred Stock [Member] | 5.75% Cumulative Convertible Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred Stock, Shares Outstanding, Beginning of Period			843,000	1,497,000
Conversion of Stock, Shares Converted			(72,600)	(24,601)
Preferred Stock, Liquidation Preference Per Share	$ 1,000		$ 1,000
Preferred Stock, Shares Outstanding, End of Period	770,000	1,472,000	770,000	1,472,000
Preferred Stock, Dividend Rate, Percentage			5.75%
Preferred Stock [Member] | 5.75% Cumulative Convertible Preferred Stock Series A [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred Stock, Shares Outstanding, Beginning of Period			476,000	1,100,000
Conversion of Stock, Shares Converted			(12,500)	(1,201)
Preferred Stock, Liquidation Preference Per Share	$ 1,000		$ 1,000
Preferred Stock, Shares Outstanding, End of Period	463,000	1,099,000	463,000	1,099,000
Preferred Stock, Dividend Rate, Percentage			5.75%
Preferred Stock [Member] | 4.50% Cumulative Convertible Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred Stock, Shares Outstanding, Beginning of Period			2,559,000	2,559,000
Conversion of Stock, Shares Converted			0	0
Preferred Stock, Liquidation Preference Per Share	$ 100		$ 100
Preferred Stock, Shares Outstanding, End of Period	2,559,000	2,559,000	2,559,000	2,559,000
Preferred Stock [Member] | 5.0% Cumulative Convertible Preferred Stock Series 2005 B [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred Stock, Shares Outstanding, Beginning of Period			1,962,000	2,096,000
Conversion of Stock, Shares Converted			(150,948)	0
Preferred Stock, Liquidation Preference Per Share	$ 100		$ 100
Preferred Stock, Shares Outstanding, End of Period	1,811,000	2,096,000	1,811,000	2,096,000
Common Stock [Member] | 5.75% Cumulative Convertible Preferred Stock [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, Conversion of Convertible Securities			7,442,156	975,488
Common Stock [Member] | 5.75% Cumulative Convertible Preferred Stock Series A [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, Conversion of Convertible Securities			1,205,923	46,018
Common Stock [Member] | 5.0% Cumulative Convertible Preferred Stock Series 2005 B [Member]
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock Issued During Period, Shares, Conversion of Convertible Securities			1,317,756