Consolidated Statements of Changes In Equity (USD $)	Total	Common Stock	Additional Paid-in Capital	8.034% Series A Cumulative Redeemable Perpetual Preferred Stock	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, 2011	$ 799,280,000	$ 412,000	$ 998,483,000	$ 0	$ 0	$ (270,021,000)	$ 72,953,000	$ (2,547,000)
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Conversion of OP Units to common stock	0	2,000	3,818,000	0	0	0	(3,820,000)	0
Issuance of common stock through exercise of options	2,756,000	1,000	2,755,000	0	0	0	0	0
Issuance of common stock through employee stock purchase plan	926,000	0	926,000	0	0	0	0	0
Compensation expenses related to stock options and restricted stock	4,465,000	0	4,465,000	0	0	0	0	0
Repurchase of common stock or Common OP Units	(43,000)	0	(43,000)	0	0	0	0	0
Adjustment For Common Unitholders In Operating Partnership	0	0	(150,000)	0	0	0	150,000	0
Shelf registration costs	(18,000)	0	(18,000)	0	0	0	0	0
Adjustment for fair market value of swap	(466,000)	0	0	0	0	0	0	(466,000)
Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs, Preferred Stock Offering Costs	(82,000)	0	(82,000)	0	0	0	0	0
Reclassification of Series A Preferred Stock	200,000,000	0	0	200,000,000	0	0	0	0
Net income available to Common Shares	33,395,000	0	0	0	0	30,504,000	2,891,000	0
Distributions	(59,437,000)	0	0	0	0	(54,273,000)	(5,164,000)	0
Stock Exchange	0	0	0	(136,144,000)	136,144,000	0	0	0
Ending Balance at Sep. 30, 2012	$ 980,776,000	$ 415,000	$ 1,010,154,000	$ 63,856,000	$ 136,144,000	$ (293,790,000)	$ 67,010,000	$ (3,013,000)