Consolidated Statement of Equity (Parenthetical) - $ / shares	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2018
Shares pursuant to dividend reinvestment plan (in shares)	0	120,347
Shares due to exercise of employee stock options and issuance of directors' deferred stock (in shares)	0	485
Issuance of partnership units pursuant to dividend reinvestment plan (in shares)	0	13,742
Dividend Distributions Payable
Distributions payable common stock, per share (in usd per share)	$ 0.53	$ 0.53
Distributions partnership units, per unit (in usd per share)	0.53	0.53
Series C Cumulative Preferred Stock | Dividend Distributions Payable
Distributions payable on preferred stock, per share (in usd per share)		42.97
Series D Cumulative Redeemable Preferred Stock | Dividend Distributions Payable
Distributions payable on preferred stock, per share (in usd per share)	38.28	$ 38.28
Series E Cumulative Redeemable Preferred Stock | Dividend Distributions Payable
Distributions payable on preferred stock, per share (in usd per share)	$ 37.50