Shareholders' Equity/Partners' Capital - Schedule of Dividends (Details) - $ / shares	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Class of Stock [Line Items]
Common shares, dividends (in dollars per share)	$ 0.66	$ 0.63	$ 1.98	$ 1.89
Common shares of beneficial interest: authorized shares (shares)	250,000,000	250,000,000	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
Convertible Preferred Stock | Series A Preferred Stock
Class of Stock [Line Items]
Preferred stock, dividend rate, per-dollar-amount (in dollars per share)	$ 0.8125	$ 0.8125	$ 2.4375	$ 2.4375
Preferred shares of beneficial interest: authorized shares (shares)	83,977	83,977	83,977	83,977
Preferred stock, dividend rate, percentage (percent)			6.50%	6.50%
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 shares of beneficial interest: authorized shares (shares)	12,000,000	12,000,000	12,000,000	12,000,000
Preferred stock, dividend rate, percentage (percent)			5.70%	5.70%
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 shares of beneficial interest: authorized shares (shares)	12,000,000	12,000,000	12,000,000	12,000,000
Preferred stock, dividend rate, percentage (percent)			5.40%	5.40%
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 shares of beneficial interest: authorized shares (shares)	12,780,000	12,780,000	12,780,000	12,780,000
Preferred stock, dividend rate, percentage (percent)			5.25%	5.25%
Class A Unit
Class of Stock [Line Items]
Convertible shares (shares)			1.9531