Consolidated Statements of Stockholders' Equity (USD $) In Thousands, except Share data, unless otherwise specified	Total	Preferred Operating Partnership	Operating Partnership	Other	Par Value	Paid-in Capital	Accumulated Other Comprehensive Deficit	Accumulated Deficit	Comprehensive Income
Balances at Dec. 31, 2008	$ 946,793	$ 29,837	$ 36,628	$ 1,558	$ 858	$ 1,130,964		$ (253,052)
Balances (in shares) at Dec. 31, 2008					85,790,331
Increase (Decrease) in Stockholders' Equity
Restricted stock grants issued	5				5
Restricted stock grants issued (in shares)					547,265
Restricted stock grants cancelled (in shares)					(21,256)
Compensation expense related to stock-based awards	3,809					3,809
Noncontrolling interests consolidated as business acquisitions	726			726
Investments from other noncontrolling interests	(615)			(615)
Repurchase of equity portion of exchangeable senior notes	(2,234)					(2,234)
Redemption of Operating Partnership units for cash	(1,908)		(1,908)
Redemption of Operating Partnership units for common stock			(3,583)		4	3,579
Redemption of Operating Partnership units for common stock (in shares)					405,501
Comprehensive income:
Net income (loss)	39,093	6,186	1,826	(896)				31,977	39,093
Change in fair value of interest rate swap	(1,111)	(11)	(44)				(1,056)		(1,111)
Total comprehensive income	37,982								37,982
Tax effect from vesting of restricted stock grants and stock option exercises	(414)					(414)
Tax effect from wind down of development program	2,539					2,539
Distributions to Operating Partnership units held by noncontrolling interests	(7,664)	(6,126)	(1,538)
Dividends paid on common stock at $0.56, $0.40 and $0.38 per share for the year ended 31st December, 2011, 2010 and 2009, respectively	(32,800)							(32,800)
Balances at Dec. 31, 2009	946,219	29,886	31,381	773	867	1,138,243	(1,056)	(253,875)
Balances (in shares) at Dec. 31, 2009					86,721,841
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon the exercise of options	5,661				5	5,656
Issuance of common stock upon the exercise of options (in shares)					484,261
Restricted stock grants issued	4				4
Restricted stock grants issued (in shares)					445,230
Restricted stock grants cancelled (in shares)					(64,010)
Compensation expense related to stock-based awards	4,580					4,580
Deconsolidation of noncontrolling interests	104			104
Investments from other noncontrolling interests	87			87
Purchase of noncontrolling interest	223			223
Redemption of Operating Partnership units for cash	(4,116)		(4,116)
Comprehensive income:
Net income (loss)	33,374	6,048	1,048	(53)				26,331	33,374
Change in fair value of interest rate swap	(4,963)	(55)	(177)				(4,731)		(4,963)
Total comprehensive income	28,411								28,411
Tax effect from vesting of restricted stock grants and stock option exercises	836					836
Tax effect from contribution of property to Taxable REIT Subsidiary	(495)					(495)
Distributions to Operating Partnership units held by noncontrolling interests	(7,479)	(6,146)	(1,333)
Dividends paid on common stock at $0.56, $0.40 and $0.38 per share for the year ended 31st December, 2011, 2010 and 2009, respectively	(34,964)							(34,964)
Balances at Dec. 31, 2010	939,071	29,733	26,803	1,134	876	1,148,820	(5,787)	(262,508)
Balances (in shares) at Dec. 31, 2010	87,587,322				87,587,322
Increase (Decrease) in Stockholders' Equity
Issuance of common stock upon the exercise of options	18,622				14	18,608
Issuance of common stock upon the exercise of options (in shares)					1,388,269
Restricted stock grants issued	2				2
Restricted stock grants issued (in shares)					226,630
Restricted stock grants cancelled (in shares)					(47,695)
Issuance of common stock, net of offering costs	112,349				53	112,296
Issuance of common stock, net of offering costs (in shares)					5,335,423
Compensation expense related to stock-based awards	5,757					5,757
Redemption of Operating Partnership units for cash	(271)		(271)
Redemption of Operating Partnership units for common stock			(2,344)		3	2,341
Redemption of Operating Partnership units for common stock (in shares)					293,641
Comprehensive income:
Net income (loss)	58,423	6,289	1,689	(4)				50,449	58,423
Change in fair value of interest rate swap	(2,237)	(22)	(66)				(2,149)		(2,237)
Total comprehensive income	56,186								56,186
Tax effect from vesting of restricted stock grants and stock option exercises	2,199					2,199
Distributions to Operating Partnership units held by noncontrolling interests	(8,098)	(6,305)	(1,793)
Distributions to other noncontrolling interests	(29)			(29)
Dividends paid on common stock at $0.56, $0.40 and $0.38 per share for the year ended 31st December, 2011, 2010 and 2009, respectively	(52,027)							(52,027)
Balances at Dec. 31, 2011	$ 1,073,761	$ 29,695	$ 24,018	$ 1,101	$ 948	$ 1,290,021	$ (7,936)	$ (264,086)
Balances (in shares) at Dec. 31, 2011	94,783,590				94,783,590