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, 2013	$ 963,205,000	$ 834,000	$ 1,021,365,000	$ 136,144,000	$ (264,083,000)	$ 69,872,000	$ (927,000)
Stock Issued During Period, Value, Conversion of Units	0	(4,000)	(4,085,000)	0	0	(4,089,000)	0
Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs	(154,000)	0	(154,000)	0	0	0	0
Increase (Decrease) in Stockholders' Equity [Roll Forward]
Issuance of common stock through employee stock purchase plan	929,000	1,000	928,000	0	0	0	0
Compensation expenses related to stock options and restricted stock	4,998,000	0	4,998,000	0	0	0	0
Stock Repurchased During Period, Value	(32,000)	0	(32,000)	0	0	0	0
Adjustment for fair market value of swap	1,068,000	0	0	0	0	0	1,068,000
Release Of Common Shares From Escrow	(1,934,000)	(1,000)	(1,933,000)	0	0	0	0
Adjustment For Common Unitholders In Operating Partnership	0	0	(345,000)	0	0	345,000	0
Net income available to Common Shares	104,206,000	0	0	6,949,000	89,328,000	7,929,000	0
Distributions	(95,742,000)	0	0	(6,949,000)	(81,585,000)	(7,208,000)	0
Ending Balance at Sep. 30, 2014	$ 976,544,000	$ 838,000	$ 1,028,912,000	$ 136,144,000	$ (256,340,000)	$ 66,849,000	$ 141,000