Equity Equity - Preferred Stock Conversion Terms Table (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Preferred Stock Conversions, Inducements	$ 41,000,000	$ 428,000,000	$ 0
5.75% Cumulative Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage	5.75%
5.75% Cumulative Convertible Preferred Stock Series A [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage	5.75%
4.50% Cumulative Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage	4.50%
5.0% Cumulative Convertible Preferred Stock Series 2005 B [Member]
Class of Stock [Line Items]
Preferred Stock, Dividend Rate, Percentage	5.00%
Preferred Stock [Member] | 5.75% Cumulative Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Conversion of Stock, Stock Issue Date	Jun. 01, 2010
Preferred Stock, Liquidation Preference Per Share	$ 1,000
Conversion of Stock, Holders Conversion Right	Any time
Preferred Stock Conversion Rate	39.6858%
Conversion of Stock, Conversion Price	$ 25.1979
Convertible Preferred Stock, Terms of Conversion	May 17, 2015
Preferred Stock Conversions, Inducements	$ 32.7573
Preferred Stock, Dividend Rate, Percentage	5.75%
Preferred Stock [Member] | 5.75% Cumulative Convertible Preferred Stock Series A [Member]
Class of Stock [Line Items]
Conversion of Stock, Stock Issue Date	May 10, 2010
Preferred Stock, Liquidation Preference Per Share	$ 1,000
Conversion of Stock, Holders Conversion Right	Any time
Preferred Stock Conversion Rate	38.3508%
Conversion of Stock, Conversion Price	$ 26.0751
Convertible Preferred Stock, Terms of Conversion	May 17, 2015
Preferred Stock Conversions, Inducements	$ 33.8976
Preferred Stock, Dividend Rate, Percentage	5.75%
Preferred Stock [Member] | 4.50% Cumulative Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Conversion of Stock, Stock Issue Date	Sep. 15, 2005
Preferred Stock, Liquidation Preference Per Share	$ 100
Conversion of Stock, Holders Conversion Right	Any time
Preferred Stock Conversion Rate	2.4561%
Conversion of Stock, Conversion Price	$ 40.7152
Convertible Preferred Stock, Terms of Conversion	September 15, 2010
Preferred Stock Conversions, Inducements	$ 52.9298
Preferred Stock [Member] | 5.0% Cumulative Convertible Preferred Stock Series 2005 B [Member]
Class of Stock [Line Items]
Conversion of Stock, Stock Issue Date	Nov. 15, 2005
Preferred Stock, Liquidation Preference Per Share	$ 100
Conversion of Stock, Holders Conversion Right	Any time
Preferred Stock Conversion Rate	2.7745%
Conversion of Stock, Conversion Price	$ 36.0431
Convertible Preferred Stock, Terms of Conversion	November 15, 2010
Preferred Stock Conversions, Inducements	$ 46.8560
Preferred Stock [Member] | Minimum [Member] | 4.50% or 5.00% (Series 2005B) Cumulative Convertible Preferred Stock [Member]
Class of Stock [Line Items]
Conversion of Stock, Company Market Trigger	250,000
Preferred Stock [Member] | Minimum [Member] | 5.75% or 5.75% (Series A) Preferred Stock [Member]
Class of Stock [Line Items]
Conversion of Stock, Company Market Trigger	25,000