Equity - Preferred Stock Outstanding and Dividends (Details) - $ / shares		3 Months Ended
	Nov. 30, 2018	Mar. 31, 2019	Dec. 31, 2018
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		20,400,000	20,400,000
6.50% Series C
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		5,000,000	5,000,000
Preferred stock, dividend rate, percentage		6.50%
Dividend per share/unit (in usd per share)		$ 0.41
6.375% Series D
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		5,000,000	5,000,000
Preferred stock, dividend rate, percentage		6.375%
Dividend per share/unit (in usd per share)		$ 0.40
6.375% Series E
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		4,400,000	4,400,000
Preferred stock, dividend rate, percentage	6.375%	6.375%
Dividend per share/unit (in usd per share)		$ 0.40
6.30% Series F
Class of Stock [Line Items]
Preferred stock, outstanding (in shares)		6,000,000	6,000,000
Preferred stock, dividend rate, percentage	6.30%	6.30%
Dividend per share/unit (in usd per share)		$ 0.39