Consolidated Statements of Stockholders' Equity (USD $) In Thousands, unless otherwise specified	Total	Preferred Stock	Common Stock	Additional Paid-In Capital	Accumulated Other Comprehensive (Income) Loss	Cumulative Net Income	Cumulative Dividends	Total Stockholders' Equity	Non-controlling Interests
Beginning Balance at Dec. 31, 2011	$ 1,004,806	$ 0	$ 779	$ 1,894,604	$ (3,332)	$ 795,951	$ (1,683,196)	$ 1,004,806
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock, net of costs	202,365		93	202,272				202,365
Common stock redemption	(68)			(68)				(68)
Stock-based compensation	3,492		3	3,489				3,492
Net income	5,535					5,465		5,465	70
Defined benefit pension plan net loss	1,240				1,240			1,240
Dividends to common stockholders ($1.20 per share)	(96,356)						(96,356)	(96,356)
Distributions to noncontrolling interests	(70)								(70)
Ending Balance at Dec. 31, 2012	1,120,944	0	875	2,100,297	(2,092)	801,416	(1,779,552)	1,120,944
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock, net of costs	220,259		83	220,176				220,259
Common stock redemption	(454)			(454)				(454)
Stock-based compensation	5,210		1	5,209				5,210
Net income	6,983					6,946		6,946	37
Defined benefit pension plan net loss	2,143				2,143			2,143
Dividends to common stockholders ($1.20 per share)	(111,571)						(111,571)	(111,571)
Distributions to noncontrolling interests	(34)								(34)
Proceeds from noncontrolling interest	1,806								1,806
Ending Balance at Dec. 31, 2013	1,245,286	0	959	2,325,228	51	808,362	(1,891,123)	1,243,477	1,809
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of stock, net of costs	76,831		31	76,800				76,831
Common stock redemption	(10,074)		(4)	(10,070)				(10,074)
Stock-based compensation	4,451		2	4,449				4,451
Net income	32,200					31,887		31,887	313
Defined benefit pension plan net loss	(2,570)				(2,570)			(2,570)
Dividends to common stockholders ($1.20 per share)	(116,371)						(116,371)	(116,371)
Distributions to noncontrolling interests	(510)								(510)
Purchase of noncontrolling interest in consolidated joint ventures	(8,189)			(6,577)				(6,577)	(1,612)
Ending Balance at Dec. 31, 2014	$ 1,221,054	$ 0	$ 988	$ 2,389,830	$ (2,519)	$ 840,249	$ (2,007,494)	$ 1,221,054