Consolidated Statement of Equity (Parenthetical) - $ / shares	3 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019
Shares pursuant to dividend reinvestment plan (in shares)	12,627	83,978	99,804	120,347
Shares due to exercise of employee stock options and issuance of directors' deferred stock (in shares)	11,745	11,745	48,772	485
Issuance of partnership units pursuant to dividend reinvestment plan (in shares)		15,101	20,041	13,742
Dividend Distributions Payable
Distributions payable common stock, per share (in usd per share)	$ 0.53	$ 0.53	$ 0.53	$ 0.53
Distributions partnership units, per unit (in usd per share)	0.53	0.53	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	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	$ 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	$ 37.50