Rental and Other Revenues; Rental Property and Other Expenses (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended																12 Months Ended
	Dec. 31, 2011		Sep. 30, 2011		Jun. 30, 2011		Mar. 31, 2011		Dec. 31, 2010		Sep. 30, 2010		Jun. 30, 2010		Mar. 31, 2010		Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Real Estate Revenue, Net [Abstract]
Contractual rents, net																	$ 420,285	$ 398,224	$ 392,360
Straight-line rental income, net																	12,828	11,349	3,644
Amortization of lease incentives																	(1,371)	(1,239)	(1,100)
Property operating expense recoveries, net																	36,105	41,736	44,462
Lease termination fees																	2,443	2,992	1,813
Fee income																	5,571	5,466	5,155
Other miscellaneous operating revenues																	6,991	2,598	1,684
Real estate revenue, net	128,730	[1]	122,086	[1]	117,057	[1]	114,979	[1]	117,330	[1]	115,528	[1]	113,765	[1]	114,503	[1]	482,852	461,126	448,018
Future minimum base rents [Abstract]
Future minimum base rents, current	448,420																448,420
Future minimum base rents, year two	409,453																409,453
Future minimum base rents, year three	352,743																352,743
Future minimum base rents, year four	297,859																297,859
Future minimum base rents, year five	247,283																247,283
Future minimum base rents, thereafter	809,042																809,042
Total future minimum base rents	2,564,800																2,564,800
Operating Costs and Expenses [Abstract]
Utilities, insurance and real estate taxes																	97,156	90,838	91,173
Maintenance, cleaning and general building																	61,700	56,877	55,840
Property management and administrative expenses																	11,624	11,203	11,702
Other miscellaneous operating expenses																	5,668	4,720	3,310
Rental property and other expenses, net																	$ 176,148	$ 163,638	$ 162,025

[1] The amounts presented may not equal to the amounts previously reported in the most recent Form 10-Qs or prior 10-K for each period as a result of discontinued operations.