Consolidated Statement of Equity (Parenthetical) - $ / shares	3 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018
Shares pursuant to dividend reinvestment plan (in shares)	105,753	99,804	120,347	201,500	85,202	69,750
Shares due to exercise of employee stock options and issuance of directors' deferred stock (in shares)	1,645	48,772	485	22,005	2,647	8,088
Issuance of partnership units pursuant to dividend reinvestment plan (in shares)	13,406	20,041	13,742	13,107	219,102	38,037
Distributions payable common stock, per share (in usd per share)	$ 0.53			$ 0.52
Dividend Distributions Payable
Distributions payable common stock, per share (in usd per share)	0.53	$ 0.53	$ 0.53	0.52	$ 0.52	$ 0.52
Distributions partnership units, per unit (in usd per share)	0.53	0.53	0.53	0.52	0.52	$ 0.52
Series C Cumulative Preferred Stock
Partial redemption of Series C Cumulative preferred stock (in shares)						30,000
Series C Cumulative Preferred Stock | Dividend Distributions Payable
Distributions payable on preferred stock, per share (in usd per share)	42.97	42.97	42.97	42.97	42.97	$ 42.97
Series D Cumulative Redeemable Preferred Stock
Issuance of Series D Cumulative preferred stock (in shares)						30,000
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	$ 38.28	$ 28.92
Common Stock
Issuance of Series D Cumulative preferred stock (in shares)	107,398	148,576	120,832
Series E Cumulative Redeemable Preferred Stock
Issuance of Series D Cumulative preferred stock (in shares)	44,000
Series E Cumulative Redeemable Preferred Stock | Dividend Distributions Payable
Distributions payable on preferred stock, per share (in usd per share)	$ 5.83