Selected Quarterly Financial Data (Unaudited) (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	1 Months Ended	3 Months Ended								12 Months Ended
	Dec. 31, 2009	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
Summary of unaudited quarterly results of operations
Total revenues		$ 241,552	$ 241,785	$ 217,938	$ 210,673	$ 200,645	$ 196,272	$ 192,930	$ 192,616	$ 911,948	$ 782,463	$ 764,461
Net income available for common shareholders		1,050	14,712	9,464	37,773	6,651	53,143	(2,669)	24,630	62,999	81,755	114,006
Net income available for common shareholders per share basic and diluted		$ 0.01	$ 0.18	$ 0.13	$ 0.52	$ 0.09	$ 0.82	$ (0.04)	$ 0.43	$ 0.81	$ 1.26	$ 2.03
Common distributions declared (in dollars per share)	$ 0.48	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 0.48		$ 2.00	$ 1.48	$ 2.40
Office
Properties reclassified to discontinued operations:
Number of properties reclassified to continued operations		7
Number of properties reclassified to discontinued operations			12			30	1
Industrial and Other
Properties reclassified to discontinued operations:
Number of properties reclassified to continued operations		20
Number of properties reclassified to discontinued operations						25
Summary of unaudited quarterly results of operations
Total revenues		$ 162,456									$ 158,688	$ 147,917
Industrial
Properties reclassified to discontinued operations:
Number of properties reclassified to continued operations		20
Number of properties reclassified to discontinued operations			1