Shareholders' Equity/Partners' Capital (Schedule of Dividends) (Details) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Class of Stock [Line Items]
Common shares, dividends (in dollars per share)	$ 0.53	$ 0.66	$ 1.85	$ 1.98
Common shares of beneficial interest: authorized shares (shares)	250,000,000		250,000,000		250,000,000
Preferred shares of beneficial interest: authorized shares (shares)	110,000,000		110,000,000		110,000,000
Common Class A
Class of Stock [Line Items]
Common shares of beneficial interest: authorized shares (shares)	250,000,000	250,000,000	250,000,000	250,000,000
Convertible Preferred Stock | Common Class A
Class of Stock [Line Items]
Common shares, dividends (in dollars per share)	$ 0.53	$ 0.66	$ 1.85	$ 1.98
Convertible Preferred Stock | Series A Preferred Stock
Class of Stock [Line Items]
Common shares, dividends (in dollars per share)	0.53	0.66	1.85	1.98
Preferred stock, dividend rate, per-dollar-amount (in dollars per share)	$ 0.8125	$ 0.8125	$ 2.4375	$ 2.4375
Preferred stock, dividend rate, percentage (percent)			6.50%	6.50%
Preferred shares of beneficial interest: authorized shares (shares)	13,402	83,977	13,402	83,977
Preferred stock, redemption price per share (in dollars per share/unit)	$ 1.9531		$ 1.9531
Redeemable Preferred Stock
Class of Stock [Line Items]
Preferred stock, liquidation preference per share (in dollars per share/unit)	25.00		25.00
Redeemable Preferred Stock | Series K Preferred Stock
Class of Stock [Line Items]
Preferred stock, dividend rate, per-dollar-amount (in dollars per share)	$ 0.3563	$ 0.3563	$ 1.0689	$ 1.0689
Preferred stock, dividend rate, percentage (percent)			5.70%	5.70%
Preferred shares of beneficial interest: authorized shares (shares)	12,000,000	12,000,000	12,000,000	12,000,000
Redeemable Preferred Stock | Series L Preferred Stock
Class of Stock [Line Items]
Preferred stock, dividend rate, per-dollar-amount (in dollars per share)	$ 0.3375	$ 0.3375	$ 1.0125	$ 1.0125
Preferred stock, dividend rate, percentage (percent)			5.40%	5.40%
Preferred shares of beneficial interest: authorized shares (shares)	13,800,000,000	13,800,000,000	13,800,000,000	13,800,000,000
Redeemable Preferred Stock | Series M Preferred Stock
Class of Stock [Line Items]
Preferred stock, dividend rate, per-dollar-amount (in dollars per share)	$ 0.3281	$ 0.3281	$ 0.9843	$ 0.9843
Preferred stock, dividend rate, percentage (percent)			5.25%	5.25%
Preferred shares of beneficial interest: authorized shares (shares)	13,800,000	13,800,000	13,800,000	13,800,000
Class A Unit | Convertible Preferred Stock | Series A Preferred Stock
Class of Stock [Line Items]
Preferred stock, redemption price per share (in dollars per share/unit)	$ 1.9531		$ 1.9531