Consolidated Statements of Changes In Equity (USD $)	Total	Common Stock	Additional Paid-in Capital	Cumulative Series C Six Point Seven Five Percent Redeemable Perpetual Preferred Stock [Member]	Distributions in Excess of Accumulated Earnings	Non-controlling interests - Common OP Units	Accumulated Other Comprehensive Loss
Beginning Balance at Dec. 31, 2012	$ 924,302,000	$ 832,000	$ 1,012,514,000	$ 136,144,000	$ (287,652,000)	$ 65,054,000	$ (2,590,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through exercise of options	248,000	1,000	247,000	0	0	0	0
Issuance of common stock through employee stock purchase plan	330,000	1,000	329,000	0	0	0	0
Compensation expenses related to stock options and restricted stock	2,716,000	0	2,716,000	0	0	0	0
Adjustment for fair market value of swap	872,000	0	0	0	0	0	872,000
Release Of Common Shares From Escrow	(1,157,000)	0	(1,157,000)	0	0	0	0
Net income available to Common Shares	62,253,000	0	0	4,640,000	52,883,000	4,730,000	0
Distributions	(50,047,000)	0	0	(4,640,000)	(41,679,000)	(3,728,000)	0
Other	(479,000)	0	(479,000)	0	0	0	0
Ending Balance at Jun. 30, 2013	$ 939,038,000	$ 834,000	$ 1,014,170,000	$ 136,144,000	$ (276,448,000)	$ 66,056,000	$ (1,718,000)