Consolidated Statement of Equity (Parenthetical) - $ / shares	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Shares pursuant to dividend reinvestment plan (in shares)	120,347	69,750
Shares due to exercise of employee stock options and issuance of directors' deferred stock (in shares)	485	8,088
Issuance of partnership units pursuant to dividend reinvestment plan (in shares)	13,742	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	0.52
Distributions partnership units, per unit (in usd per share)	0.53	$ 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
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	$ 28.92