Consolidated Statement of Equity (Parenthetical)	9 Months Ended
	Sep. 30, 2017 $ / shares shares
Shares pursuant to dividend reinvestment plan (in shares) | shares	183,020
Shares due to exercise of employee stock options and issuance of directors' deferred stock (in shares) | shares	98,511
Issuance of partnership units pursuant to dividend reinvestment plan (in shares) | shares	95,755
Distributions payable common stock, per share (in usd per share)	$ 1.53
Dividend Distributions Payable
Distributions payable common stock, per share (in usd per share)	0.51
Distributions partnership units, per unit (in usd per share)	0.51
Dividend Distributions Payable | Series C Preferred Stock
Distributions payable on Series C preferred stock, per share (in usd per share)	$ 42.97