Partners' Capital of the Operating Partnership (Details) (USD $) In Thousands, except Share data, unless otherwise specified	6 Months Ended		12 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Partners' Capital of the Operating Partnership [Abstract]
Underwritten public offering of common stock, shares	6,037,500
Proceeds from issuance of common stock after deducting underwriting discounts and commissions and offering expenses	$ 221,015	$ 299,847
Issuance of common units, units (Note 8)	6,037,500
Common units in Operating Partnership	58,464,412	52,296,219	52,349,670
Common general partnership interest in operating partnership	97.10%	96.70%	96.80%
Common limited partnership interest rate	2.90%	3.30%	3.20%
Limited partnership units outstanding	1,718,131	1,723,131	1,723,131