Segment Disclosure (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012 Segments	Sep. 30, 2011
Segment Reporting Information [Line Items]
Number of Reportable Segments			1
Consolidated net income
Operating revenues	$ 104,293	$ 86,399	$ 293,800	$ 243,371
Unallocated Expenses Income [Abstract]
General and administrative expenses	(8,727)	(6,355)	(26,745)	(20,355)
Acquisition-related expenses	(556)	(1,163)	(3,897)	(2,829)
Depreciation and amortization	(44,109)	(33,275)	(116,832)	(88,969)
Interest income and other net investment (losses) gains	330	30	703	272
Interest expense	(19,854)	(22,896)	(60,172)	(62,671)
(LOSS) INCOME FROM CONTINUING OPERATIONS	(665)	(3,391)	1,535	(3,744)
Income from discontinued operations	3,187	17,681	81,936	26,373
Net income	2,522	14,290	83,471	22,629
Office Properties [Member]
Unallocated Expenses Income [Abstract]
Income from discontinued operations	(470)	14,413	72,572	18,746
Non-Reportable Segment - Industrial Properties [Member]
Unallocated Expenses Income [Abstract]
Income from discontinued operations	$ 3,657	$ 3,268	$ 9,364	$ 7,627