Equity - Preferred Stock and Dividends (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock, shares outstanding, beginning of period (in shares)	5,563,458
Preferred stock, shares outstanding, end of period (in shares)	5,563,458	5,563,458
Loss on exchange of preferred stock	$ 0	$ 17	$ 0
5.75% cumulative convertible preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock, shares outstanding, beginning of period (in shares)	770,000	770,000	770,000
Preferred stock, shares outstanding, end of period (in shares)	770,000	770,000	770,000
Preferred stock, dividend rate	5.75%
5.75% cumulative convertible preferred stock series A
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock, shares outstanding, beginning of period (in shares)	423,000	463,000	463,000
Preferred stock, shares outstanding, end of period (in shares)	423,000	423,000	463,000
Preferred stock, dividend rate	5.75%
4.50% cumulative convertible preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock, shares outstanding, beginning of period (in shares)	2,559,000	2,559,000	2,559,000
Preferred stock, shares outstanding, end of period (in shares)	2,559,000	2,559,000	2,559,000
Preferred stock, dividend rate	4.50%
5.00% cumulative convertible preferred stock series 2005 B
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock, shares outstanding, beginning of period (in shares)	1,811,000	1,811,000	1,811,000
Preferred stock, shares outstanding, end of period (in shares)	1,811,000	1,811,000	1,811,000
Preferred stock, dividend rate	5.00%
Preferred stock | Preferred stock, exchanged for shares of common stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock exchanges (in shares)	(100)	(40,000)	0
Preferred stock | 5.75% cumulative convertible preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock exchanges (in shares)		0
Preferred stock | 5.75% cumulative convertible preferred stock series A
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock exchanges (in shares)		(40,000)
Stock issued, conversion (in shares)		40,000
Interest rate, stated percentage		5.75%
Preferred stock | 4.50% cumulative convertible preferred stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock exchanges (in shares)		0
Preferred stock | 5.00% cumulative convertible preferred stock series 2005 B
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Preferred stock exchanges (in shares)		0
Common Stock | Preferred stock, exchanged for shares of common stock
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Stock issued, conversion (in shares)	0	51,839	0