Equity - Preferred Stock Table (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 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, 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, 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, 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, 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