Consolidated Statements of Equity (Parenthetical) - $ / shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Shares issued pursuant to dividend reinvestment plan (in shares)	430,462,000	572,928	266,011
Shares due to exercise of employee stock options and issuance of directors' deferred stock (in shares)	61,571	43,150	152,758
Issuance of partnership units pursuant to dividend reinvestment plan (in shares)	60,936,000	107,433	111,351
Issuance of partnership units (in shares)	60,936	284,113
Series C Cumulative Preferred Stock
Redemption of Series C Cumulative preferred stock (in shares)	42,000	30,000
Series D Cumulative Redeemable Preferred Stock
Issuance of Series D Cumulative preferred stock (in shares)		30,000
Series E Cumulative Redeemable Preferred Stock
Issuance of Series D Cumulative preferred stock (in shares)	44,000
Dividend declared
Distributions payable common stock and partnership units (in usd per share)	$ 0.53	$ 0.53	$ 0.52
Dividend declared | Series C Cumulative Preferred Stock
Distributions payable on preferred stock (in usd per share)		42.97	$ 42.97
Dividend declared | Series D Cumulative Redeemable Preferred Stock
Distributions payable on preferred stock (in usd per share)	38.28	$ 38.28
Dividend declared | Series E Cumulative Redeemable Preferred Stock
Distributions payable on preferred stock (in usd per share)	$ 37.50