Quarterly Financial Data (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2010		Sep. 30, 2010		Jun. 30, 2010		Mar. 31, 2010		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Quarterly Financial Data [Line Items]
Rental and other revenues	$ 128,730	[1]	$ 122,086	[1]	$ 117,057	[1]	$ 114,979	[1]	$ 117,330	[1]	$ 115,528	[1]	$ 113,765	[1]	$ 114,503	[1]	$ 482,852	$ 461,126	$ 448,018
Income from continuing operations	12,850	[1]	5,402	[1]	14,143	[1]	12,106	[1]	11,153	[1]	8,501	[1]	39,811	[1]	11,445	[1]	44,501	70,910	46,458
Income from discontinued operations	0	[1]	2,842	[1]	291	[1]	337	[1]	246	[1]	272	[1]	238	[1]	637	[1]	3,470	1,393	15,236
Net income	12,850		8,244		14,434		12,443		11,399		8,773		40,049		12,082		47,971	72,303	61,694
Net (income) attributable to noncontrolling interests in the Operating Partnership	(595)		(366)		(623)		(507)		(501)		(366)		(1,933)		(520)		(2,091)	(3,320)	(3,197)
Net (income) attributable to noncontrolling interests in consolidated affiliates	(201)		(249)		(182)		(123)		(204)		148		(215)		(214)		(755)	(485)	(11)
Dividends on Preferred Stock	(627)		(627)		(1,622)		(1,677)		(1,677)		(1,677)		(1,677)		(1,677)		(4,553)	(6,708)	(6,708)
Excess of Preferred Stock redemption/repurchase over carrying value	0		0		(1,895)		0										(1,895)	0	0
Net income available for common stockholders	11,427		7,002		10,112		10,136		9,017		6,878		36,224		9,671		38,677	61,790	51,778
Earnings Per Share, Basic [Abstract]
Income from continuing operations available for common stockholders (in dollars per share)	$ 0.16		$ 0.06		$ 0.14		$ 0.14		$ 0.13		$ 0.10		$ 0.51		$ 0.13		$ 0.49	$ 0.84	$ 0.55
Income from discontinued operations available for common stockholders (in dollars per share)	$ 0.00		$ 0.04		$ 0.00		$ 0.00		$ 0.00		$ 0.00		$ 0.00		$ 0.01		$ 0.05	$ 0.02	$ 0.21
Net income available for common stockholders	$ 0.16		$ 0.10		$ 0.14		$ 0.14		$ 0.13		$ 0.10		$ 0.51		$ 0.14		$ 0.54	$ 0.86	$ 0.76
Earnings Per Share, Diluted [Abstract]
Income from continuing operations available for common stockholders (in dollars per share)	$ 0.16		$ 0.06		$ 0.14		$ 0.14		$ 0.13		$ 0.10		$ 0.50		$ 0.13		$ 0.49	$ 0.84	$ 0.55
Income from discontinued operations available for common stockholders (in dollars per share)	$ 0.00		$ 0.04		$ 0.00		$ 0.00		$ 0.00		$ 0.00		$ 0.00		$ 0.01		$ 0.05	$ 0.02	$ 0.21
Net income available for common stockholders	$ 0.16		$ 0.10		$ 0.14		$ 0.14		$ 0.13		$ 0.10		$ 0.50		$ 0.14		$ 0.54	$ 0.86	$ 0.76
Continuing Operations Reconciliation To Previous Reported Amounts [Abstract]
Rental and other revenues, as reported			122,086		117,057		115,592		117,865		115,528		113,765		115,054
Discontinued operations			0		0		(613)		(535)		0		0		(551)
Rental and other revenues, as adjusted	128,730	[1]	122,086	[1]	117,057	[1]	114,979	[1]	117,330	[1]	115,528	[1]	113,765	[1]	114,503	[1]	482,852	461,126	448,018
Income from continuing operations, as reported			5,402		14,143		12,443		11,399		8,501		39,811		11,694
Discontinued operations			0		0		(337)		(246)		0		0		(249)
Income from continuing operations, as adjusted	12,850	[1]	5,402	[1]	14,143	[1]	12,106	[1]	11,153	[1]	8,501	[1]	39,811	[1]	11,445	[1]	44,501	70,910	46,458
Income from discontinued operations, as reported			2,842		291		0		0		272		238		388
Additonal discontinued operations from properties sold subsequent to the respective reporting period			0		0		337		246		0		0		249
Income from discontinued operations, as adjusted	$ 0	[1]	$ 2,842	[1]	$ 291	[1]	$ 337	[1]	$ 246	[1]	$ 272	[1]	$ 238	[1]	$ 637	[1]	$ 3,470	$ 1,393	$ 15,236

[1] The amounts presented may not equal to the amounts previously reported in the most recent Form 10-Qs or prior 10-K for each period as a result of discontinued operations.