Consolidated Statement of Equity (Parenthetical) - $ / shares	3 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2019
Shares pursuant to dividend reinvestment plan (in shares)	14,525,000	12,627,000	83,978,000	105,753,000	99,804,000	120,347,000
Shares due to exercise of employee stock options and issuance of directors' deferred stock (in shares)	984,000	3,109,000	11,745,000	1,645,000	48,772,000	485,000
Issuance of partnership units pursuant to dividend reinvestment plan (in shares)	13,108,000		15,101,000	13,406,000	20,041,000	13,742,000
Common stock, shares issued (in shares)	23,358,208						23,231,240
Dividend Distributions Payable
Distributions payable common stock, per share (in usd per share)	$ 0.53	$ 0.53	$ 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	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	42.97
Series D Cumulative Redeemable Preferred Stock
Cumulative redeemable preferred stock, shares issued (in shares)	30,000,000						30,000,000
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	$ 38.28
Series E Cumulative Redeemable Preferred Stock
Cumulative redeemable preferred stock, shares issued (in shares)	44,000,000			44,000,000			44,000,000
Series E Cumulative Redeemable Preferred Stock | Dividend Distributions Payable
Distributions payable on preferred stock, per share (in usd per share)	$ 37.50	$ 37.50	$ 37.50	$ 5.83