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)
Stock Issued During Period, Value, Conversion of Units	0	0	160,000	0	0	(160,000)	0
Stock Issued During Period, Value, Stock Options Exercised	248,000	1,000	247,000	0	0	0	0
Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs	(504,000)	0	(504,000)	0	0	0	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through employee stock purchase plan	526,000	1,000	525,000	0	0	0	0
Compensation expenses related to stock options and restricted stock	4,332,000	0	4,332,000	0	0	0	0
Adjustment For Common Unitholders In Operating Partnership	0	0	6,780,000	0	0	(6,780,000)	0
Adjustment for fair market value of swap	1,233,000	0	0	0	0	0	1,233,000
Release Of Common Shares From Escrow	(2,360,000)	0	(2,360,000)	0	0	0	0
Net income available to Common Shares	97,189,000	0	0	6,951,000	82,755,000	7,483,000	0
Distributions	(75,117,000)	0	0	(6,951,000)	(62,518,000)	(5,648,000)	0
Stock Issued During Period, Value, Acquisitions	9,686,000	0	0	0	0	9,686,000	0
Ending Balance at Sep. 30, 2013	$ 959,535,000	$ 834,000	$ 1,021,694,000	$ 136,144,000	$ (267,415,000)	$ 69,635,000	$ (1,357,000)