Consolidated Statement of Stockholders' Equity (Parenthetical)	6 Months Ended
	Jun. 30, 2017 $ / shares shares
Shares pursuant to dividend reinvestment plan (in shares) | shares	97,289
Due to exercise of employee stock options and issuance of directors' deferred stock (in shares) | shares	75,971
Issuance of partnership units pursuant to dividend reinvestment plan (in shares) | shares	79,734
Distributions payable common stock, per share (in usd per share)	$ 1.02
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