Earnings Per Unit of the Operating Partnership (Computations of Basic and Diluted Earnings Per Unit) (Details) (USD $) In Thousands, except Share data	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010
Earnings Per Unit of the Operating Partnership
Net income available to unitholders	$ 3,720	$ 4,308	$ 9,375	$ 8,736
Less: net income allocable and distributions in excess of earnings to participating securities	(291)	(211)	(601)	(415)
Net income attributable to unitholders - basic	3,429	4,097	8,774	8,321
Net income attributable to unitholders - basic	3,429	4,097	8,774	8,321
Add: net income allocable and distributions in excess of earnings to dilutive participating securities		211		415
Net income attributable to unitholders	$ 3,429	$ 4,308	$ 8,774	$ 8,736
Basic	132,782,072	112,582,265	132,742,123	106,890,664
Unvested units		1,373,812		1,407,471
Diluted	132,782,072	113,956,077	132,742,123	108,298,135
Net income per share attributable to common stockholders, basic and diluted	$ 0.03	$ 0.04	$ 0.07	$ 0.08