Condensed Consolidated Statements of Equity - USD ($) $ in Thousands	Total	Preferred Stock	Common Stock	Additional Paid-In Capital	Treasury Stock	Distributions in Excess of Net Income	Stockholders’ Investment	Nonredeemable Noncontrolling Interests
Beginning balance at Dec. 31, 2016	$ 2,514,240	$ 6,867	$ 403,747	$ 3,407,430	$ (148,373)	$ (1,214,114)	$ 2,455,557	$ 58,683
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	4,858					4,751	4,751	107
Common stock issued pursuant to:
Common stock offering, net of issuance costs	211,825		25,000	186,825			211,825
Stock-based compensation	(701)		231	(932)			(701)
Spin-off of Parkway, Inc.	404					404	404
Common stock redemption by unit holders	0		251	1,766			2,017	(2,017)
Amortization of stock options and restricted stock, net of forfeitures	489		(3)	492			489
Contributions from nonredeemable noncontrolling interests	630							630
Distributions to nonredeemable noncontrolling interest	(491)							(491)
Common dividends	(48,738)					(48,738)	(48,738)
Ending balance at Mar. 31, 2017	2,682,516	6,867	429,226	3,595,581	(148,373)	(1,257,697)	2,625,604	56,912
Common stock issued pursuant to:
Cumulative effect of change in accounting principle	22,329					22,329	22,329
Beginning balance at Dec. 31, 2017	2,825,111	6,867	430,350	3,604,776	(148,373)	(1,121,647)	2,771,973	53,138
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Net income	16,406					16,043	16,043	363
Common stock issued pursuant to:
Stock-based compensation	(759)		232	(991)			(759)
Amortization of stock options and restricted stock, net of forfeitures	542		(9)	551			542
Distributions to nonredeemable noncontrolling interest	(399)							(399)
Common dividends	(27,315)					(27,315)	(27,315)
Ending balance at Mar. 31, 2018	$ 2,835,915	$ 6,867	$ 430,573	$ 3,604,336	$ (148,373)	$ (1,110,590)	$ 2,782,813	$ 53,102